OBJECTIVE: To assess the understanding of and attitudes towards randomized clinical trials amongst patients attending oncology out-patient clinics. DESIGN: Cross-sectional survey. SUBJECTS: Patients attending medical oncology out-patient clinics at a Sydney teaching hospital. MAIN OUTCOME: Patients' willingness to participate in a randomized clinical trial. RESULTS: Sixty consecutive patients were surveyed. The mean age was 55.2 (SD 14) years. Eighty-eight per cent of respondents thought that patients should be asked to participate in trials testing new treatments, however, only a third would consider participating in a randomized trial themselves. If a trial was endorsed by an independent cancer information service such as the NSW Cancer Council, 72% of respondents would be more likely to participate. Knowledge about randomized trials was not high. Respondents scored a median of 3 out of 7 (interquartile range, 2-4) correct answers to a series of questions about randomized trials. Patients willing to participate in a randomized trial were more likely to perceive the doctor favourably (P = 0.05), less likely to perceive trials as experimental (P = 0.05) and less likely to perceive trials as representing an inconvenience or loss of control (P = 0.09). CONCLUSIONS: Understanding amongst patients of the need for and mechanisms of randomized clinical trials is not good. This may contribute to the difficulties investigators face in seeking consent for clinical trials. Evaluation of new strategies to educate the public and patients about randomized trials is needed. Involvement of consumers in the design and conduct of clinical trials and evaluation of strategies to improve doctors' communication of clinical trial information is also required.