Background. The effect of early antiretroviral therapy (ART) on sexual behaviors and HIV-1 transmission risk has not been documented beyond the specific population of stable serodiscordant couples.Methods. Based on a behavioral study nested in a randomized controlled trial (Temprano-ANRS12136) of early ART, we compared proportions of risky sex (unprotected sex with a partner of negative/unknown HIV status) reported 12 months after inclusion between participants randomized to initiate ART immediately ('early ART') or according to WHO criteria ('standard ART'). Group-specific HIV-transmission rates were estimated based on sexual behaviors and viral load-specific per-act HIV-1 transmission probabilities. Their ratio was computed to estimate the protective effect of early ART.Results. Among 957 participants (baseline CD4: 478/mm(3)), 46.0% reported sexual activity in the past month, 41.5% of them with non-cohabiting partners. Proportion of risky sex was 10.0% vs. 12.8%, respectively, in participants on early vs. standard ART (p=0.17). Accounting for sexual behaviors and viral load, the estimated protective effect of early ART was 90% (95%CI 81-95%).Conclusion. Twelve months after inclusion, patients on early and standard ART reported similar sexual behaviors. Early ART decreased the estimated risk of HIV transmission by 90%, suggesting a major prevention benefit among both stable and casual partners.