ObjectivesTo ascertain the level of acceptance of a prophylactic HIV vaccine trial in high-risk HIV-seronegative heterosexual cohorts of men, and women in Mombasa, Kenya, and to assess the anticipated effects of participation on risk behavior.MethodsStandardized questionnaire administered to a convenience sample of commercial sex workers, and trucking company employees enrolled in prospective cohort studies.ResultsNinety-six per cent of respondents believed that HIV was a major problem in Kenya, and 86% of men, and 94% of women perceived themselves at risk. One hundred per cent of women, and 84% of men expressed interest in participation in an HIV vaccine trial, after explanation of the experimental nature of the vaccine, double-blind placebo-controlled design, prolonged follow-up, and potential change in serostatus. Seventeen per cent of men, and 9% of women anticipated an increase in risk behavior as a result of participation.ConclusionThe majority of individuals in two high-risk cohorts were interested in participating in Phase III efficacy trials of HIV vaccines. A significant minority anticipated an increase in risk behavior, which emphasizes the need for intensive counseling, and education throughout a vaccine trial.