Individuals who use illicit stimulants, primarily cocaine and methamphetamine, engage in substantial amounts of HIV-related sexual risk behaviors when under the influence. This paper presents the idea that reductions in stimulant use consequent to drug treatment makes stimulant drug treatment an important HIV prevention tool for this high-risk population. Data are presented to describe many of the HIV-related sexual risks reported by out-of-treatment methamphetamine users and by cocaine and methamphetamine abusers at treatment entry and six months post treatment entry. Overall, our findings demonstrate that following initiation of a treatment episode, stimulant abusers demonstrate significant and relevant reductions in HIV-related sexual behaviors, primarily by reducing the number of sexual partners. Reasons for why stimulant treatment corresponds to HIV transmission risk reductions and suggestions for implementing prevention messages in treatment settings are offered.