OPINION OF THE COURT

Robert Gibbs applied for Disability Insurance Benefits and Supplemental Security Income, asserting an April 6, 2001, onset date of disability. After a hearing, the Administrative Law Judge (“ALJ”) issued a decision finding Gibbs disabled within the meaning of the Social Security Act, 42 U.S.C. § 423(d)(1)(A), beginning on April 6, 2001. Gibbs appealed to the District Court, claiming a disability onset date of November 1, 1999. 1 The District Court affirmed the agency’s decision that the onset date of Gibbs’ disability was April 6, 2001, not November 1,1999.

On appeal to this court, Gibbs argues that, under Social Security Ruling (SSR) 83-20, his disability may be inferred back to November 1, 1999. He contends that if he was limited to less than sedentary work after April 6, 2001, it can be inferred, due to the progressive nature of his medical ailments, that he was limited to sedentary or light work as of November 1, 1999.

Under SSR 83-20, the ALJ is required to call a medical expert in cases such as this where the onset date of disability must be inferred. SSR 83-20; see Newell v. Comm’r of Soe. Sec., 347 F.3d 541, 549 (3d Cir. 2003) (stating that “[t]he ALJ should have consulted a medical advisor to help him infer the onset date as required by SSR 83-20”). While the determination of Gibbs’ disability onset date involves consideration of his allegations, work history, and medical and other relevant evidence, any judgment made must have a legitimate medical basis. SSR 83-20.

In this case, the ALJ properly followed SSR 83-20’s requirements by calling a medical expert to help determine the onset date of Gibbs’ disability. Gibbs provided no medical evidence that supported an onset date of November 1, 1999. We there *195 fore will affirm the well-reasoned opinion and the order of the district court.