BACKGROUND: The effect of early initiation of antiretroviral therapy (ART; ie, at CD4(+) T-cell counts >350 cells/mm(3)) on sexual behaviors and human immunodeficiency virus type 1 (HIV) transmission risk has not been documented in populations other than HIV-serodiscordant couples in stable relationships. METHODS: On the basis of data from a behavioral study nested in a randomized, controlled trial (Temprano-ANRS12136) of early ART, we compared proportions of risky sex (ie, unprotected sex with a partner of negative/unknown HIV status) reported 12 months after inclusion between participants randomly assigned to initiate ART immediately (hereafter, "early ART") or according to ongoing World Health Organization criteria. Group-specific HIV transmission rates were estimated on the basis of sexual behaviors and viral load-specific per-act HIV transmission probabilities. The ratio of transmission rates was computed to estimate the protective effect of early ART. RESULTS: Among 957 participants (baseline median CD4(+) T-cell count, 478 cells/mm(3)), 46.0% reported sexual activity in the past month; of these 46.0%, sexual activity for 41.5% involved noncohabiting partners. The proportion of subjects who engaged in risky sex was 10.0% in the early ART group, compared with 12.8% in the standard ART group (P = .17). After accounting for sexual behaviors and viral load, we estimated that the protective effect of early ART was 90% (95% confidence interval, 81%-95%). CONCLUSION: Twelve months after inclusion, patients in the early and standard ART groups reported similar sexual behaviors. Early ART decreased the estimated risk of HIV transmission by 90%, suggesting a major prevention benefit among seronegative sex partners in stable or casual relationships with seropositive individuals.