Behaviors associated with transmission of the human immunodeficiency virus (HIV) were measured in a sample of 60 adults with a severe and persistent mental illness (SPMI). Results revealed that 68% had sex in the last year; 13% of men and 30% of women reported two or more male partners, and 24% of men also reported two or more female partners. Condom use was inconsistent. Sex partners were often met in a psychiatric clinic or bar, and a substantial number were injection drug users or known to be non-monogamous. Overall, 48% of men and 37% of women reported at least one risk factor. Hypothesized psychological antecedents of HIV-related risk behavior were also measured, including knowledge, motivation for risk reduction, and self-efficacy regarding risk-reduction. Many participants were misinformed regarding HIV-transmission and risk reduction. Motivational indices indicated that attitudes toward condoms were slightly positive, and that social norms were generally supportive of condom use. However, participants tended to rate themselves at only slight risk for infection, undermining their motivation for condom use. Participants indicated only modest levels of self-efficacy in situations requiring sexual assertiveness. These findings, coupled with the elevated seroprevalence of HIV among persons having a SPMI, point to the need for risk assessment and counseling by mental health care providers.