The purpose of this cross-sectional study was to examine relationships between belief in vaccine receipt, motivations for trial participation, and side effects in phase 1 vaccine trials. Anonymous questionnaires were completed by 125 active vaccine volunteers at two vaccine evaluation sites. Participants believing they had received the vaccine reported more side effects (p < .01), were less likely to report knowing someone with HIV/AIDS as a motivation for trial participation (p < .01), and endorsed greater concern about becoming HIV-infected as motivation for participation (p < .05). Results indicate that inferences made by trial participants in vaccine trials should be identified and addressed, and that greater efforts for maintaining the blinded nature of vaccine trials and educating trial participants about the meaning of side effects are warranted.