This article empirically examines how healthy volunteers evaluate and make sense of the risks of phase I clinical drug trials. This is an ethically important topic because healthy volunteers are exposed to risk but can gain no medical benefit from their trial participation. Based on in-depth qualitative interviews with 178 healthy volunteers enrolled in various clinical trials, we found that participants focus on myriad characteristics of clinical trials when assessing risk and making enrolment decisions. These factors include the short-term and long-term effects; required medical procedures; the type of trial, including its design, therapeutic area of investigation, and dosage of the drug; the amount of compensation; and trust in the research clinic. In making determinations about the study risks, participants rely on information provided during the consent process, their own and others’ experiences in clinical trials, and comparisons among studies. Our findings indicate that the informed consent process succeeds in communicating well about certain types of risk information while simultaneously creating lacunae that are problematically filled by participants through their collective experiences and assumptions about risk. We discuss the ethical implications of these findings and make recommendations for improving the consent process in healthy volunteer trials.