This article compares the differences in HIV prevalence and risk behaviors in singly diagnosed patients with substance abuse problems and dually diagnosed patients with comorbid psychiatric and substance abuse problems. The National Treatment Improvement Evaluation Study (NTIES) was used to conduct a cross-sectional study on 6593 persons treated for substance abuse disorders. Logistic regression using hierarchal linear modeling (HLM) was utilized to compare risk behaviors of singly and dually diagnosed persons. Compared with singly diagnosed patients, dually diagnosed patients were more likely to share a needle, have sex for money or gifts, have sex with an intravenous drug user, and report being raped. Patients dually diagnosed with psychiatric and substance abuse disorders are an especially high-risk group for HIV infection, in part due to a higher likelihood of engaging in high-risk behavior, and should be targeted for more intense HIV interventions.