OBJECTIVES: The study evaluated the relative impact of HIV risk reduction interventions for adults with severe mental illness living in the inner city. METHODS: A total of 104 chronically mentally ill men and women were interviewed to determine sexual risk behavior over the past month and to assess HIV risk-related psychological characteristics, including their knowledge about risk behavior, their belief in their ability to change their behavior, their perceptions of peer and social norms about safer sex, their expectancies about the outcomes of these changes, and their perceived barriers to condom use. Participants were then randomly assigned to one of three conditions: a single AIDS education session, a seven-session cognitive-behavioral HIV risk reduction group intervention, or a seven-session group intervention that combined the cognitive-behavioral intervention with training to act as a risk reduction advocate to friends (advocacy training). Individuals were reinterviewed three months after completion of the intervention. RESULTS: Although all participants exhibited change at follow-up in some risk-related psychological characteristics and sexual risk behaviors, participants who received the cognitive- behavioral intervention that included the advocacy training reported greater reductions in rates of unprotected sex and had fewer sexual partners at follow-up. CONCLUSIONS: HIV prevention interventions that teach risk reduction skills and then encourage participants to advocate behavior change to others appear to strengthen participants' capacity to change their behavior to reduce HIV risk, even those from a disenfranchised group such as severely mentally ill adults.