OBJECTIVE: Studies have reported an increased prevalence of HIV infection among psychiatric patients. Inaccurate HIV knowledge is included as a factor in the increased risk of HIV infection in the mentally ill, but few studies have looked specifically at this factor. The aims of the study were to determine the knowledge of HIV and its transmission among adult psychiatric patients at Weskoppies Hospital and to determine the relationship between HIV knowledge and HIV risk behaviour. METHOD: Structured interviews were conducted with 113 consenting adult patients at Weskoppies Hospital. They were divided into three groups according to their length of hospital stay. The structured interview included questions about demographic data, the diagnoses and the AIDS Risk Behaviour Knowledge Test (AIDS-KT). Scores of 13 out of 13 represented accurate knowledge of HIV (level I); scores of 10-12 represented good knowledge (level II); scores of ≤9 represented poor knowledge (level III). RESULTS: A total of 104 patients (92%) demonstrated excellent knowledge of HIV and its transmission (levels I and II). There was no significant linear association between HIV knowledge and risk-behaviour scores (Pearson’s correlation coefficient r=-0.11). CONCLUSION: The presence of high-risk behaviours despite good HIV-related knowledge in this group of patients, leads us to think that knowledge alone will not limit HIV risk behaviours. For this reason, educational programmes should not be limited to interventions that simply increase knowledge about HIV infection but should extend to clinical factors, including patients’ motivation and readiness to change their behaviour.