This paper reviews reports of the effects of clinical evaluations on physician awareness and behavior. Among the reviewed papers, there are 28 studies of the effect of specific randomized clinical trials (RCTs) on 19 different medical practices. After analyzing these 28 papers, there remain two (7%) where the RCT has clear implications for practice, where the pattern of practice reported quantitatively over time conforms fully to the RCT findings, where the RCT preceded the change in the pattern of practice, and where findings from the RCT differ from the results of other forms of evaluation. The majority of the reviewed papers do not support an inference that RCTs have a strong, direct influence in changing established clinical practices. Clinical evaluation is one among many factors bearing on changes in medical practices. Improving the care of patients requires both improved methods of evaluation and more effective translation of the results of evaluation into practice. Evaluations are likely to exert a greater impact on medical practices if they are buttressed by attention to other controllable factors, like channels of communication and environmental constraints and incentives, that affect the adoption and abandonment of medical practices.