Examined the prevalence and correlates of human immunodeficiency virus (HIV)/acquired immunodeficiency syndrome (AIDS) risk behaviors in a large sample of severely mentally ill (SMI) patients. Risk levels were correlated with demographic factors, diagnosis, symptom severity, trauma history, post-traumatic stress disorder (PTSD), substance use disorder (SUD), and sexual orientation. 275 SMI clients (aged 18-60 yrs) from urban and rural settings were assessed regarding HIV/ AIDS risk behaviors, and hypothesized risk factors. Patients exhibited substantial levels of risky behavior, particularly sexual risk. Correlates of increased risk included SUD, trauma, male homosexual orientation, younger age, and symptom severity. Structural equation modeling identified SUD and sexual orientation as the primary determinants of both drug and sexual risk behavior. The authors conclude that specific illness related variables appear to have less impact on risk behavior among people with SMI than previously hypothesized. Substance abuse prevention and treatment may be the most effective means of reducing HIV risk in this population.