We conducted an HIV seroincidence study among high-risk men who have sex with men in Rio de Janeiro to determine whether this population would be suitable for vaccine and non-vaccine HIV prevention studies. There were 34 HIV seroconversions during the follow-up period, for a seroincidence of 3.1 per 100 person-years. The incidence among the youngest members of our cohort was over 8%, an extraordinarily high transmission rate for a homosexual male population. We have used the sensitive/less sensitive ELISA assay to identify a high-risk heterosexual population for intervention studies in Brazil. The estimated seroincidence was 1.9 and 2.8 per 100 person-years among heterosexual females and males, respectively. We are presently conducting a study to investigate behavior changes among 202 participants of a post-sexual-exposure chemoprophylaxis (PEP) study. During a median follow-up of 24 months, there were 11 HIV seroconversions, 10 of which were among non-PEP users. Additionally, we are conducting a phase 2 HIV vaccine trial of the ALVAC-HIV vector vCP1452 alone and in combination with a subunit vaccine, MN rgp120, in healthy adult volunteers. The primary goal of this trial is to define the immunogenicity and confirm the safety of the most promising vector-based vaccine alone and in combination with a recombinant envelope protein vaccine with an established safety and immunogenicity profile. A phase 3 HIV vaccine trial is planned to begin in the next 18-24 months.