MEMORANDUM OPINION; ORDER THEREON
Plaintiff Jacqueline Stack (“Plaintiff’) filed a Complaint on May 14, 2003, seeking review of the decision of the Commissioner of Social Security (“Commissioner”) denying her application for a period of disability and disability benefits under Title II of the Social Security Act (“Act”). Plaintiff *1176seeks reversal of the Commissioner’s decision, or in the alternative, asks the Court to remand the matter for further administrative proceedings.
Plaintiffs application for disability benefits was filed on November 16, 1998 (Administrative Record [“AR”] 94-96), alleging an onset date of August 22, 1996. (AR 102.) She alleged a disabling condition of chronic severe depression and arthritis. (AR 102.) Her application was denied initially (AR 71-73), and she requested reconsideration (AR 74), which was also denied. (AR 74-76.) Plaintiff then requested a hearing before an Administrative Law Judge(“ALJ”) (AR 86), and filed an Appointment of Representative, designating Dennis H. Devermont, an attorney, as her representative. (AR 34.) The hearing was held in Pasadena, California on March 23, 2000. Plaintiff testified, as did a vocational expert (“VE”), Rheta King. (AR 35-69.) Following the hearing, the ALJ issued a decision on May 22, 2000, finding Plaintiff to be not disabled. (AR 25.)
In this action, Plaintiff filed her Motion for Summary Judgment (“Motion”) on December 2, 2003. The Commissioner filed a Cross-Motion for Summary Judgment (“Cross-Motion”) on February 6, 2004, and on March 22, 2004, Plaintiff flied her Reply (“Reply”). The parties have consented to proceed before Magistrate Judge Victor B. Kenton, pursuant to 28 U.S.C. § 636(c). The matter is now ready for decision.
ISSUE RAISED
In her Motion, Plaintiff raises the following issue:
(1) The ALJ failed to properly consider Plaintiffs obesity in the sequential disability analysis (Motion at 4).
DISCUSSION
The Court, pursuant to 42 U.S.C. § 405(g), has the authority to review the Commissioner’s decision denying Plaintiffs disability benefits to determine whether her findings are supported by substantial evidence and whether the Commissioner used the proper legal standards in reaching her decision. Meanel v. Apfel, 172 F.3d 1111, 1113 (9th Cir.1999); Reddick v. Chater, 157 F.3d 715, 720 (9th Cir.1998).
A claimant is “disabled” for the purpose of receiving benefits under the Social Security Act if he is unable to engage in any substantial gainful activity due to an impairment which has lasted, or is expected to last, for a continuous period of at least twelve months. 42 U.S.C. §§ 423(d)(1)(A), 1382c(a)(3)(A); 20 CFR §§ 404.1505(a), 416.905(a). “The claimant bears the burden of establishing a prima facie case of disability.” Roberts v. Shalala, 66 F.3d 179, 182 (9th Cir.1995); Smolen v. Chater, 80 F.3d 1273, 1289 (9th Cir.1996).
Regulations promulgated by the Commissioner establish a five-step sequential evaluation process to be followed by the ALJ in a disability case. 20 C.F.R. §§ 404.1520, 416.920. In the First Step, the ALJ must determine whether the claimant is currently engaged in substantial gainful activity; if so, a finding of nondisability is made and the claim is denied. 20 C.F.R. §§ 404.1520(b), 416.920(b). If the claimant is not currently engaged in substantial gainful activity, in the Second Step, the ALJ must determine whether the claimant has a severe impairment or combination of impairments significantly limiting him from performing basic work activities; if not, a finding of nondisability is made and the claim is denied. 20 C.F.R. §§ 404.1520(c), 416.920(c). If the claimant has a severe impairment, in the Third Step, the ALJ must compare the impairment to those impairments in the Listing of Impairments (“Listing”), 20 C.F.R. § 404, Subpart P, App. 1; if the impairment meets or equals an impairment in the Listing, disability is conclusively *1177presumed and benefits are awarded. 20 C.F.R. § 404.1520(d), 416.920(d). When the claimant’s impairment does not meet or equal an impairment in the Listing, in the Fourth Step, the ALJ must determine whether the claimant has sufficient residual functional capacity despite the impairment or various limitations to perform his past work; if so, a finding of nondisability is made and the claim is denied. 20 C.F.R. §§ 404.1520(e), 416.920(e). When the claimant shows an inability to perform past relevant work, a prima facie case of disability is established and, in Step Five, the burden shifts to the Commissioner to show the claimant can perform other work that exists in significant numbers in the national economy. 20 C.F.R. §§ 404.1520(f), 416.920(f).
Following this sequential evaluation process, the ALJ found that at Step One, Plaintiff had not engaged in substantial gainful activity since the alleged onset date of disability. (AR, Finding 1.) At Step Two, Plaintiff was found to have severe impairments consisting of osteoarthritis of the lumbar spine and left knee, combined with a depressive disorder. (Id., Finding 2.) At Step Three, the ALJ found that Plaintiffs medical impairments did not, either individually or in combination, meet or equal any of the listed impairments. (Id., Finding 4.) At Step Four, the ALJ concluded that the Plaintiff could not return to her past relevant work. (Id., Finding 7.) At Step Five, he determined that Plaintiffs impairments did not prevent her from performing a significant number of jobs which exist in the national economy, given her residual functional capacity (“RFC”) and vocational profile. (Id., Finding 8.) The ALJ found that Plaintiff was not under a disability under the definition provided in the Act. (Id., Finding 9.)
THE ALJ FAILED TO GIVE DUE CONSIDERATION TO PLAINTIFF’S OBESITY
Plaintiffs treating physicians at the Ne-phrology Medical Associates diagnosed her with osteoarthritis of the lumbar spine and left knee. (AR 228-29, 230.) Sue Folk Smith, Ph.D., diagnosed Plaintiff with a depressive disorder. (AR 176.) In addition, there was consistent evidence of obesity. On September 18, 1998, Plaintiffs treating physician, George E. Fischmann, M.D. FACP, recommended a weight loss plan based on his conclusion that Plaintiff had “an obviously deteriorating course with increasing weight that is exacerbating her cholesterol and arthritis.” (AR 229.) Dr. Fischmann noted that Plaintiff had gained 20 pounds since her last visit and had reached a weight of 203 pounds. (AR 228.) On November 12, 1998, treating physician, Russell J. Vergess, M.D., also recommended a weight loss program. (AR 230.) On January 17, 2000, examining physician Jeremy Schweiger, M.D., diagnosed Plaintiff with obesity, recording her weight at 207 pounds on a five foot five frame. (AR 486-87.)1
Based on this record, the ALJ found that Plaintiff suffered from severe impairments consisting of osteoarthritis of the lumbar spine and left knee. (AR 24.) The ALJ also found that Plaintiff had a depressive order but concluded that alcohol abuse was a material contributing factor to her mental symptoms. (AR 23.) The ALJ concluded at Step Three that Plaintiffs impairments did not meet, nor were they medically equivalent to, a listed impairment. (AR 24.)
Plaintiff argues that the ALJ failed to acknowledge her obesity or the effect of her obesity on the sequential disability *1178analysis. (Motion at 4.) The Commissioner argues the ALJ properly assessed all of the evidence, including the significance of Plaintiffs obesity, and properly concluded that Plaintiffs impairments were not sufficient to form a presumption of disability. (Motion at 4.)
“In determining whether an individual’s impairments are of sufficient medical severity that such ... could be the basis of eligibility under this section, the Secretary shall consider the combined effect of all the individual’s impairments without regard to whether any such impairment, if considered separately, would be of such severity.” Celaya v. Halter, 332 F.3d 1177, 1181 (9th Cir.2003), quoting 42 U.S.C. § 423(d)(2)(B). At Step Three of the sequential analysis, “the ALJ must explain adequately his evaluation of alternative tests and the combined effects of the impairments.” Celaya, 332 F.3d at 1182, quoting Marcia v. Sullivan, 900 F.2d 172, 176 (9th Cir.1990). When there is evidence of obesity, the ALJ must determine the effect of the claimant’s obesity upon her other impairments, her ability to work, and her general health. Celaya, 332 F.3d at 1182.
In Celaya, the pro se claimant was diagnosed with diabetes and hypertension. She also was categorized as extremely obese, in that her Body Mass Index (“BMI”) exceeded the criterion level of 40.2 The Ninth Circuit held that although the claimant had not explicitly raised her obesity as a disabling factor, it nevertheless should have been included in the multiple impairment analysis because it was implicitly raised in her report of symptoms and it was clear from the record. The Celaya court noted the ALJ has a special duty to develop the record and assure claimant’s interests are considered. Id. at 1183. The court reasoned that fulfillment of that duty requires an assessment of the claimant’s obesity even though not expressly raised by the claimant herself. Id.
The ALJ breached his duty to develop the record and assure Plaintiffs interests were considered by failing to acknowledge Plaintiffs obesity and to weigh its interactive effect on her other existing impairments. Although there was no mention of Plaintiffs obesity during the administrative hearing, consistent evidence of obesity is present throughout the record. See Celaya, 332 F.3d at 1182; see also Clifford v. Apfel, 227 F.3d 863, 873 (7th Cir.2000) (“While [the claimant] did not claim obesity as an impairment when filing her Disability Report, the evidence should have alerted the ALJ that [she] had another relevant impairment that could contribute to the cumulative effect of her other impairments.”). As noted above, Plaintiffs treating physician diagnosed her as being obese, her weight fluctuated between 200 and 229 pounds, and her increasing weight and recommended weight loss plan were recorded numerous times. (See AR 231, 238, 261, 279, 416, 487.) While Plaintiff here was represented at the administrative hearing, the Celaya court expressly stated that the ALJ’s duty to develop the record with respect to obesity applies “even when the claimant is represented by counsel.” Celaya, 332 at 1183, quoting Brown v. Heckler 713 F.2d 441, 443 (9th Cir.1983) (citations omitted).
*1179The Commissioner argues that Celaya is inapplicable to Plaintiffs case because the ALJ acknowledged Plaintiffs obesity and accepted functional capacity assessments from medical sources who also had considered her obesity. (Cross-Motion at 3.) In support of this argument, the Commissioner cites to page 19 of the Administrative Record wherein the ALJ states that “Doctors ... recommended a weight loss program.” (AR 19.) Because the ALJ is required to adequately explain his evaluation and the combined effect of a claimant’s impairments, see Marcia, 900 F.2d at 176, the Court finds that the ALJ’s passing reference to a recommended weight loss program falls far short of acknowledging a diagnosis of obesity and weighing the effect of that obesity on Plaintiffs severe impairments (osteoarthritis and depressive disorder). See Barrett v. Barnhart, 355 F.3d 1065, 1068-69 (7th Cir.2004) (relying on Celaya and noting that “[e]ven if [the claimant’s] arthritis was not particularly serious in itself, it would interact with her obesity to make standing for two hours at a time more painful than it would be for a person who was either as obese as she or as arthritic as she but not both”).
The Commissioner also contends that any failure to properly assess Plaintiffs obesity in conjunction with her other impairments was Plaintiffs fault because she failed to sustain her burden of proving that she was more physically limited by her obesity. (Opposition at 3-4.) This argument is defeated by Celaya; it was the ALJ’s burden to develop the record and assess her multiple impairments once he was put on notice of Plaintiffs obesity. See Celaya, 332 F.3d at 1182-83 (“The ALJ was responsible for determining the effect of Celaya’s obesity upon her other impairments.”) Even assuming arguendo Plaintiff had such a burden, the record shows that she did present evidence that her treating physician had concluded her arthritis worsened with her increased weight. (See AR 229 (“This is a patient with an obviously deteriorating course with increasing weight that is exacerbating her cholesterol and arthritis.”).)
The ALJ had a duty to assess the impact of Plaintiffs undisputed obesity on the various steps of the sequential evaluation process. Because this did not occur, the ALJ committed legal error in assessing Plaintiffs disability claim. See Celaya, 332 F.3d at 1181-83. Accordingly, the Court reverses the ALJ’s decision and remands Plaintiffs claim for reconsideration and a new hearing consistent with this opinion and the Ninth Circuit’s decision in Celaya. On remand, the ALJ shall consider and discuss in the Step Three analysis, the effect of Plaintiffs obesity as a medically determinable impairment and its effect on Plaintiffs other impairments.3 The ALJ also shall consider and discuss how Plaintiffs obesity factors into her residual functional capacity.
ORDER
IT IS THEREFORE ORDERED that (1) Plaintiffs Motion for Summary Judgment is GRANTED in part and DENIED in part; (2) the Commissioner’s Cross-*1180Motion for Summary Judgmént is DENIED; and (3) the matter is remanded for further proceedings consistent with this Memorandum Opinion.