To determine whether continuing medical education affects the quality of clinical care, we randomly allocated 16 Ontario family physicians to receive or not receive continuing-education packages covering clinical problems commonly confronted in general practice. Over 4500 episodes of care, provided before and after study physicians received continuing education, were compared with preset clinical criteria and classified according to quality. Although objective tests confirmed that the study physicians learned from the packages, there was little effect on the overall quality of care. When the topics were of relatively great interest to the physicians, the control group (who did not receive the packages) showed as much improvement as did the study group. When the topics were not preferred, however, the documented quality of care provided by study physicians rose (P less than 0.05) and differed from that provided by control physicians (P = 0.01). Finally, there was no spillover effect on clinical problems not directly covered by the program. In view of the trend toward mandatory continuing education and the resources expended, it is time to reconsider whether it works.