Background: Phase I trials test the safety and tolerability of investigational drugs and often use healthy volunteers as research participants. Adverse events (AEs) are collected in part through participants’ self-reports of any symptoms they experience during the trial. In some cases, experiencing AEs can result in trial participation being terminated. Because of the economic incentives underlying their motivation to participate, there is concern that healthy volunteers routinely fail to report AEs and thereby jeopardize the validity of the trial results. Methods: We interviewed 131 U.S. healthy volunteers about their experiences with AEs, including their rationales for reporting or failing to report symptoms. Results: We found that participants have three primary rationales for their AE reporting behavior: economic, health-oriented, and data integrity. Participants often make decisions about whether to report AEs on a case-by-case basis, evaluating what effects reporting or not reporting might have on the compensation they receive from the trial, the risk to their health, and the results of the particular clinical trial. Participants’ interpretations of clinic policies, staff behaviors, and personal or vicarious experiences with reporting AEs also shape reporting decisions. Conclusions: Our findings demonstrate that participants’ reporting behavior is more complex than previous portraits of healthy volunteers have suggested. Rather than finding participants who were so focused on the financial compensation that they were willing to subvert trial results, our study indicates that participants are willing in most cases to forgo their full compensation if they believe not reporting their symptoms jeopardizes their own safety or the validity of the research.