Antisocial personality disorder (ASP) is common in male substance abusers and may be associated with increased human immunodeficiency virus (HIV) risk behaviors. In this study, 91 male substance abusers were recruited from the community, and 42% met diagnostic criteria for ASP. Although ASP and non-ASP subjects demonstrated equivalent knowledge about HIV, subjects with ASP participated in more risky behaviors. On a lifetime measure of drug risk behaviors, ASP subjects reported higher rates of intravenous drug use (IVDU), frequency of needle-sharing, and number of equipment-sharing partners and lower rates of needle-cleaning. On a measure of past-month risk behaviors, ASP subjects reported higher rates of IVDU and lower rates of needle-cleaning. Subjects with ASP also reported greater participation in lifetime sexual risk behaviors, including number of sexual partners and frequency of anal sex. These findings suggest that clients entering substance abuse treatment programs should be screened for ASP, and clients identified with ASP should be provided risk-reduction interventions early in treatment.