Little systematic information is available concerning the advantages and disadvantages of participation in a clinical trial from the patients' point of view. Surveys were undertaken among participants in the Beta-Blocker Heart Attack Trial (BHAT) and the Aspirin Myocardial Infarction Study (AMIS) to obtain data on these perceptions. In AMIS, an open-ended personal interview format was employed. For BHAT, a questionnaire partially based on hypotheses generated in AMIS, was mailed out. Results from the two studies suggested that patients in both trials felt that the additional medical monitoring, the opportunity for a "second opinion," and the reassurance received were more important benefits than actual physical improvement. Altruistic motivations were high in both studies. Frequency of perceived disadvantages was low, centering mainly around transportation problems and clinic waiting time. The large majority of patients indicated that they would volunteer for similar research in the future. The results are discussed in the context of the available literature and of the possibilities for extensions of this line of research.