To demonstrate the extent to which physicians change their specialty, randomized samples of the graduating classes of 1960, 1964, and 1968 were studied with respect to their self-designated specialties in 1971 and 1976. Of the 2,046 physicians in the three samples, 333 (16%) changed their specialty between 1971 and 1976. It was 8% for the 1960 cohort, 11% for 1964, and 29% for 1968. Of all the changes, 78% were from one specialty practice to another or back to a formal residency in a different specialty; 22% changed from one specialty residency to either practice or another residency in a different specialty. Between 1971 and 1976, a total of 127 (16%) of 783 primary care physicians changed their specialty. The magnitude of this change must be considered in planning for distribution of physicians by specialty.