Objective: The study examined behavioral and cognitive factors that may place mentally ill persons at increased risk of infection by the human immunodeficiency virus (HIV). Methods: Sixty-one patients consecutively admitted to a psychiatric inpatient unit of a public general hospital completed questionnaires focused on their knowledge about AIDS and their sexual practices. They also participated in a structured interview to assess how their beliefs about health related to changes in health behaviors. Responses of the psychiatric patients, of whom 54.5 percent were men and 61.5 percent were black, were categorized by diagnosis (schizophrenia, bipolar affective disorder, and depression) and were compared with responses of a control group of 32 patients with no identified psychiatric disorder who had been treated in the hospital's medical emergency room. Results: Trends in the data suggested that the psychiatric patients were more likely than the control subjects to engage in high-risk sexual behaviors. Psychiatric patients with different diagnoses appeared to engage in different kinds of high-risk behaviors. Whereas control subjects seemed inclined to change their behaviors as their knowledge about HIV increased, schizophrenic patients appeared willing to change their behavior only if they believed their behavior could really make a difference in whether they would become infected. Conclusions: The relationship between specific psychiatric symptoms, knowledge about HIV, and factors influencing behavior need to be more thoroughly examined so that clinicians can develop interventions to reduce the risk of HIV infection in mentally ill persons.