Clinician-delivered prevention interventions offer an opportunity to integrate risk-reduction counseling as a routine part of medical care. The HIV Intervention for Providers study, a randomized controlled trial, developed and tested a medical provider HIV prevention training intervention in 4 northern California HIV care clinics. Providers were assigned to either the intervention or control condition (usual care). The intervention arm received a 4-hour training on assessing sexual risk behavior with HIV-positive patients and delivering risk-reduction-oriented prevention messages to patients who reported risk behaviors with HIV-uninfected or unknown-status partners. To compare the efficacy of the intervention versus control on transmission risk behavior, 386 patients of the randomized providers were enrolled. Over six-months of follow-up, patients whose providers were assigned the intervention reported a relative increase in provider-patient discussions of safer sex (OR = 1.49; 95% CI = 1.06 to 2.09), assessment of sexual activity (OR = 1.60; 95% CI = 1.05 to 2.45), and a significant decrease in the number of sexual partners (OR = 0.49, 95% CI = 0.26 to 0.92). These findings show that a brief intervention to train HIV providers to identify risk and provide a prevention message results in increased prevention conversations and significantly reduced the mean number of sexual partners reported by HIV-positive patients.