Sex under the influence of drugs or alcohol is associated with high-risk sexual behavior. Heterosexual men (n = 505) in substance abuse treatment completed a computer-administered interview assessing sexual risk behaviors. Most men (73.3%) endorsed sex under the influence in the prior 90 days, and 39.1% endorsed sex under the influence during their most recent sexual event. Sex under the influence at the most recent event was more likely to involve anal intercourse, sex with a casual partner, and less condom use. Patients might benefit from interventions targeting sexual behavior and substance use as mutual triggers. (Am J Addict 2010;00:1-9).