An HIV seroincidence study was conducted to identify a high-risk population for HIV prevention trials. Inclusion criteria were male gender, homosexual behavior, age between 18 and 50 years, and negative HIV serostatus; 862 study subjects were screened and 753 were enrolled and observed during follow-up for a mean of 1.5 years. In this population, 34 people had HIV seroconversions for an overall annual seroincidence of 3.1% (95% confidence interval [CI], 2.1%-4.1%). Among study subjects <20 years old, annual incidence was 8.4% (95% CI, 1.7%-15%). Independent risk factors for seroconversion were age <25 years (p = .01), hepatitis B core antibody seropositivity (p > .01), sex at first encounter in the preceding 6 months (p = .11), and a history of gonorrhea or condyloma in the 6 months before seroconversion (p = .04 and p = .08, respectively). At enrollment, 85% of the eventual seroconverters said they would participate in a vaccine trial; all agreed to participate when told there would be a placebo arm. Follow-up rates were 97%, 91%, and 88% at 6, 12, and 18 months, respectively. The HIV-1 subtype was B for each of the first 17 seroconverters. These data demonstrate the suitability of this cohort for HIV prevention trials, based on high HIV incidence and retention rates, and a willingness to participate in such trials.