This study is among the first to examine knowledge about human immunodeficiency virus (HIV) and behavioral risks for HIV transmission among veterans with severe mental illness (SMI), a group at high risk for HIV infection. This study examined associations between accuracy of HIV knowledge, risk behaviors, and clinical and demographic characteristics in a sample of male veteran psychiatric inpatients diagnosed with SMI (N = 353). Results showed high rates of inaccurate HIV knowledge, with > 40% of patients demonstrating some inaccuracies, particularly those related to the progression and symptoms of acquired immunodeficiency syndrome. Inaccurate HIV knowledge was associated with older age, minority status, education level, marital status, no homelessness within the previous 6 months, and no reported history of illicit intranasal drug use. There is a need for more effective HIV prevention interventions for persons with SMI.