We tested the hypothesis that the risk or discomfort associated with a clinical trial influence patients' decisions to participate. Simultaneously, we evaluated factors likely to influence patients' decisions such as understanding of the risk and discomfort associated with the study, patient age, educational level, and psychological status. With IRB approval, participants, who believed they were being asked to participate in a real trial, were presented one of three sham protocols: no risk or pain (Control, n = 48), pain but no risk (Pain, n = 51), or risk but no pain (Risk, n = 51). Patients were debriefed at the end of the interview. Our major outcome measures were (a) understanding risk or pain associated with the proposed studies, (b) the extent to which patients felt pressured to participate, and (c) willingness to participate. Whereas understanding was similar in all groups (Control, 68%; Pain, 67%; and Risk, 72%), willingness to participate differed significantly (Control, 64%; Pain, 35%; Risk, 26%; P < 0.001). Patients who understood the level of risk or pain associated with the protocols were twice as likely to participate than those who did not (49% versus 24%; P = 0.003). Nine percent agreed to participate in the risky or painful protocols without understanding the risks involved. Patients who felt pressured did not agree to participate. Thus, the consent process protected patients, although for unexpected reasons. Understanding was poor, but patients who did not understand the risks or pain involved or who felt pressured rarely consented. Consequently, relatively few patients unknowingly agreed to participate in risky or painful studies.