To determine the willingness of populations at high risk of HIV-1 infection to participate in HIV vaccine efficacy trials, determine factors influencing decision-making, and evaluate knowledge levels of vaccine trial concepts. Cross-sectional study. HIV-1-negative homosexual men, male and female injecting drug users and non-injecting women at heterosexual risk were recruited in eight cities in the United States (n=4892). A substantial proportion of the study population (77%) would definitely (27%) or probably (50%) be willing to participate in a randomized vaccine efficacy trial. Increased willingness was associated with high-risk behaviors, lower education level, being uninsured or covered by public insurance, and not having been in a previous vaccine preparedness study. Altruism and a desire for protection from the vaccine were major motivators for participation. Major concerns included positive HIV-1 antibody test due to vaccine, safety of the vaccine, and possible problems with insurance or foreign travel. Baseline knowledge of vaccine trial concepts was low. It is likely that high-risk volunteers will be willing to enroll in HIV vaccine efficacy trials. A variety of participant and community educational strategies are needed to address participant concerns, and to ensure understanding of key concepts prior to giving consent for participation.