We investigated the rate of HIV infection in patients at the St Ann's psychiatric hospital in Trinidad and Tobago, and identified the demographic and clinical variables associated with infection. Patients admitted to the hospital were tested for HIV when details of their sexual history suggested that they might be at high risk of infection. We examined hospital records for the 1991-1995 period. During that time a total of 1,227 patients were tested, of whom 84 (6.9%) were confirmed positive for HIV. Since there was a total of 11,203 admissions over the period, the minimum infection rate for all patients was 0.75%. Among the high-risk group tested, being positive for HIV was associated with age (p = 0.01) and ethnicity (p = 0.003). The highest rates of infection were in the 15-44-year age group while the rates in patients of African and mixed ethnicity were higher than in East Indians. When the underlying diagnoses were examined, the highest rates were found in patients with substance abuse problems, especially those who abused cocaine (p < 0.001). Patients who were VDRL reactive were also more likely to be HIV positive than other patients (p < 0.001). These findings indicate that psychiatric patients are at greater than average risk for HIV infection. Mental health specialists may need to be trained in sexual health counselling to facilitate preventive intervention for this high-risk group.