Unrealistic optimism is a bias that leads people to believe, with respect to a specific event or hazard, that they are more likely to experience positive outcomes and/or less likely to experience negative outcomes than similar others. The phenomenon has been seen in a range of health-related contexts—including when prospective participants are presented with the risks and benefits of participating in a clinical trial. In order to test for the prevalence of unrealistic optimism among participants of early-phase oncology trials, we (...) conducted a survey with patients over 18 years of age who were enrolled in a phase I, phase I/II, or phase II clinical cancer trial in the New York City area between August 2008 and October 2009. Participants in our study were asked to compare their own chances of experiencing a range of risks and benefits related to the trial they were enrolled in with the chances of the other trial participants. We found a significant optimistic bias in their responses. Respondents tended to overestimate the benefits of the trial they were enrolled in and underestimate its risks. In addition, we found no significant relationship between respondents’ understanding of the trial’s purpose and how susceptible they were to unrealistic optimism. Our findings suggest that improving the consent process for oncology studies requires more than addressing deficits in understanding. (shrink)