Plaintiff-appellant brought suit under section 205(g) of the Social Security Act, 42 U.S.C. § 405(g), seeking judicial review of the Secretary’s final decision denying claimant’s1 applications for disability benefits and Supplemental Security Income (SSI). The district court declined to reverse the Secretary’s determination.
Claimant, a 49-year-old truck driver, filed applications for disability benefits and SSI in 1977 alleging that he had been disabled and unable to work since December 4, 1975; on that date he had been exposed to carbon monoxide from a leaky exhaust system in the truck he was driving, and this triggered symptoms that lead to a diagnosis of heart disease. The applications were denied initially and upon reconsideration. After a hearing before an Administrative Law Judge (ALJ) at which the reports of several doctors were admitted and plaintiff, claimant, his representative, and a vocational expert testified, the ALJ concluded that claimant was not disabled within the meaning of the Social Security Act; although claimant was unable to return to work as a truck driver, he retained the residual functional capacity to perform sedentary work. The Appeals Council approved this decision on May 3, 1978. After receiving notification that claimant had died April 3, 1978, following surgery for replacement of an aortic valve, the Appeals Council reconsidered the matter, but concluded that there was no basis for vacating its previous decision.2 The AU’s decision *77thus stood as the Secretary’s final determination.
According to claimant’s testimony, his physical problems included the following: dizzy spells with nausea which occurred at least two times a week and lasted for several minutes; shortness of breath at rest as well as after exertion; constant headaches and pain in the left rib cage; numbness and weakness in his left arm, hand, and fingers; poor vision; nervous tension; and chronic diarrhea. He could stand for only 15 minutes and sit for only 45 minutes before experiencing dizziness. He occasionally helped his wife wash dishes, but otherwise did very little housework. He had no hobbies and watched television often, but was unable to read more than a paragraph at a time. He could walk three- and-a-half blocks to the post office, but had been advised by his doctor not to do any lifting. Claimant also testified: that he was unable to sit during an eight hour work day; that he could not do any fine work with his hands due to his poor vision, but could see to do soldering on larger items; and that he could not do piece work or work under pressure because of his nervous tension. He took prenestrol, oreopin, valium, and darvon every day, and nitroglycerine as necessary for shortness of breath and chest pain. Claimant’s wife testified that he had a seizure whenever he tried to exert himself; that she had to help him out of the bathtub; and that the treating physician, Dr. Robert H. Bickford, had told her always to accompany her husband because of his seizures.
The medical evidence, consisting of hospital records and of two physicians’ reports and their responses to inquiries from the Social Security Administration, spans the period from December 1975 to February 1978. The reports indicate that claimant had a serious and deteriorating heart condition, with eventual aortic valve replacement surgery anticipated. (There was some suggestion that the replacement valve was a new device, and it would be better to delay non-emergency surgery until there was more experience with it.) The physicians reported at various times that claimant could handle work less strenuous than over the road truck driving, and that he was totally incapacitated and would remain so until after surgery.3
*78The only question before the district court, and on appeal, is whether there is substantial evidence in the record taken as a whole to support the Secretary’s conclusion that claimant was capable of sedentary work. Plaintiff argues that substantia] evidence is lacking because the AU could have reached his decision only by ignoring all of claimant’s uncontradicted testimony concerning his subjective experience of pain and physical and emotional disability, as well as certain statements from both Dr. Bickford and Dr. Emmanuel. As an indication that subjective evidence was ignored, plaintiff points to the AU’s use of two hypothetical questions propounded to the vocational expert. The AU first asked the expert whether claimant could perform gainful work if he had all the pain and limitations he claimed, and the expert replied that claimant could not. The AU then asked whether claimant could work if he was unable to stand or walk for prolonged periods and was subject to anginal attacks upon exertion, but was able to sit for long periods and had no problems with vision, tension or numbness in his fingers. The expert was explicitly instructed to exclude from consideration claimant’s ulcer and the testimony concerning nervousness. There was also no mention of any effects of medication. (Both hypotheticals assumed claimant’s age, education, and work experience.) To this second question, the vocational expert responded that claimant would be capable of sedentary work {e.g., electronics assembly), and that such work was available in the area.
In finding that claimant was not disabled, the AU apparently relied on the answer to the second hypothetical, implicitly rejecting the evidence that claimant was incapable of sedentary work. The magistrate, to whom the district court referred the case, recommended reversal of the AU on the ground that he had failed to explain why he rejected all the evidence consistent with a finding of disability. In rejecting this recommendation, the district court noted that a reviewing court’s task is much easier when the AU articulates his reasons for rejecting or accepting particular testimony. The court nevertheless concluded that because the AU recited the subjective evidence consistent with the first hypothetical, he had not ignored it, and would be presumed to have rejected it on credibility grounds; there was some record evidence, e.g., a clinic report that claimant’s vision was corrected by glasses, that provided a basis for questioning claimant’s credibility. (This does not account for the rejection of certain of the physicians’ reports.) It is on the assumption that claimant’s testimony must have been rejected for good reason — a not unreasonable assumption given the guidance heretofore furnished to the district courts — that we cannot agree with the district court.
While we must defer to the credibility determinations of the fact-finder, we must be sure that the AU has indeed made a credibility determination. This conclusion follows from the definition of “substantial evidence” set forth in Universal Camera Corp. v. N.L.R.B., 340 U.S. 474, 71 S.Ct. 456, 95 L.Ed. 456 (1951), in which the Supreme Court vacated a decision enforcing a Board order, finding it insufficient that the evidence supporting the Board’s order was substantial when considered in isolation. The Court held that the evidence supporting the agency’s decision must be substantial “when viewed in the light that the record in its entirety furnishes, including the body of evidence opposed to the [agency’s] view.” Id. at 488, 71 S.Ct. at 465. While there may be strong grounds upon which the AU rejected claimant’s evidence in this case, as the district judge presumed, we cannot say on the basis of the record that such a conclusion is self-evident. In the absence of an explicit and reasoned rejection of an entire line of evidence, the remaining evidence is *79“substantial” only when considered in isolation. It is more than merely “helpful” for the ALJ to articulate reasons (e.g., lack of credibility) for crediting or rejecting particular sources of evidence. It is absolutely essential for meaningful appellate review. As the Third Circuit put it in Cotter v. Harris, 642 F.2d 700, 705 (3d Cir.1981), when the AU fails to mention rejected evidence, “the reviewing court cannot tell if significant probative evidence was not credited or simply ignored.”
This court is not unmindful of the heavy and unique burden placed upon ALJs in Social Security Act cases. We emphasize that we do not require a written evaluation of every piece of testimony and evidence submitted. However, a minimal level of articulation of the AU’s assessment of the evidence is required in eases in which considerable evidence is presented to counter the agency’s position.
In the absence of such articulation here,, the judgment of the district court is reversed with instructions to remand the case to the Social Security Administration for further proceedings consistent with this opinion.4
Reversed and Remanded.