OBJECTIVE: Various studies have reported increased prevalence of HIV infection among psychiatric patients. Psychiatric patients are under-evaluated in terms of their HIV risk behaviour. The study sought to establish the prevalence of HIV risk behaviour and determine the association between risk behaviour and demographic and clinical variables. METHOD: Participants were 113 consenting adult in-patients. A structured interview was conducted with each participant. A total risk behaviour score was calculated. From the risk score, three risk behaviour categories were identified: 0 = no risk; 1 to 3 = medium risk; 4 to 9 = high risk. Associations between HIV risk behaviour and demographic and clinical variables were analysed. RESULTS: Of the 113 participants, 68% were men and 32% women. The mean age was 38. Forty-five per cent were sexually active and 48% fell into the “no-risk group”, 29% in the “medium-risk” group, and 23% in the “high-risk” group. Female patients with a history of treatment for sexually transmitted disease and a diagnosis of personality disorder were associated with being sexually abused. Having multiple sex partners was associated with diagnoses of substance-related disorders and cognitive disorders. Sex with someone known for less than 24 hours was associated with long-term hospitalisation and diagnoses of cognitive and personality disorders. CONCLUSION: The study confirmed that mentally ill patients are vulnerable and may be victimised. The study also suggests that mental illness may impair appreciation of consequences and lead to high-risk behaviour for contracting HIV. Special care should be taken to protect female patients in psychiatric institutions.