Background: We evaluated whether routine biannual sexually transmitted disease (STD) testing coupled with brief risk-reduction counseling reduces STD incidence and high-risk behaviors. Methods: The SUN study is a prospective observational HIV cohort study conducted in 4 US cities. At enrollment and every 6 months thereafter, participants completed a behavioral survey and were screened for STDs, and if diagnosed, were treated. Medical providers conducted brief risk-reduction counseling with all patients. Among men who have sex with men (MSM), we examined trends in STD incidence and rates of self-reported risk behaviors before and after exposure to the risk-reduction intervention. The "preintervention" visit was the study visit that was at least 6 months after enrollment STD screening and treatment and at which the participant was first exposed to the intervention. The "postintervention" visit was 12 months later. Results: Among 216 MSM with complete STD and behavioral data, median age was 44.5 years; 77% were non-Hispanic white; 83% were on highly active antiretroviral treatment; 84% had an HIV RNA level <400 copies/mL and the median CD4 (cluster of differentiation 4) count was 511 cells/mm. Twelve months after first exposure to the risk-reduction intervention, STD incidence declined from 8.8% to 4.2% (P = 0.041). Rates of unprotected receptive or insertive anal intercourse with HIV-positive partners increased (19% to 25%, P = 0.024), but did not change with HIV-negative partners or partners of unknown HIV status (24% to 22%, P = 0.590). Conclusions: STD incidence declined significantly among HIV-infected MSM after implementing frequent, routine STD testing coupled with risk-reduction counseling. These findings support adoption of routine STD screening and risk-reduction counseling for HIV-infected MSM.