OBJECTIVES To assess the extent to which HIV-negative cohort study participants would be willing to participate (WTP) in future HIV vaccine trials, to explore enrollment into an ongoing phase 3 HIV vaccine trial, and to assess changing WTP in such trials over time. METHODS The Vanguard Project is a prospective study of gay and bisexual men in the greater Vancouver region, British Columbia, Canada. Sociodemographic characteristics, sexual risk behavior, beliefs around HIV, and reasons for not participating in the AIDSVAX B/B trial were collected from self-administered questionnaires. Contingency table analysis compared subjects who were WTP with subjects who were not WTP. Logistic regression analyses identified possible predictors of WTP. A subset analysis was conducted to assess changes in WTP in 2001 versus 1997. RESULTS Of 440 respondents, 214 (48.6%) were WTP, and 97 (22.0%) were not WTP. Those WTP were disadvantaged, sexually risky, and had a high-perceived HIV risk (all p <.05). Reasons for not participating in the AIDSVAX B/B trial included fear of health problems and having missed the deadline for enrollment (all p < 0.05). Multivariate analysis revealed that having had a regular sex partner (adjusted odds ratio, 0.48 [confidence interval, 0.25-0.92]) was a negative predictor whereas having a high-perceived HIV risk (adjusted odds ratio, 5.35 [confidence interval, 1.57-18.25]) was a positive predictor of WTP. Comparing WTP in 2001 with that in 1997, 24% of 100 participants who had been previously WTP were now not WTP. CONCLUSION Improving community and participant knowledge about preventive HIV vaccine trials may help ensure informed consent. However, whether informing potential participants will reverse or contribute to the declining trend in WTP observed in this cohort warrants further investigation.