Background:Antiretroviral treatment (ART) reduces HIV infectiousness but the effect of early ART on sexual behaviour is unclear.Methods:We assessed, within the START randomized trial that enrolled HIV-positive adults with CD4⁺ cell count greater than 500cells/μl, the effect of early (immediate) versus deferred ART on: condomless sex with HIV-serodifferent partners (CLS-D); all condomless sex (CLS); HIV transmission-risk sex (CLS-D-HIV risk, defined as CLS-D and: not on ART or started ART <6 months ago or viral load greater than 200 copies/ml or no viral load in past 6 months), during 2-year follow-up. Month-12 CLS-D (2010-2014) was the primary outcome.Results:Among 2562 MSM, there was no difference between immediate and deferred arms in CLS-D at month 12 [12.6 versus 13.1%; difference (95% CI):-0.4% (-3.1 to 2.2%), P=0.75] or month 24, or in CLS. Among 2010 heterosexual men and women, CLS-D at month 12 tended to be higher in the immediate versus deferred arm [10.8 versus 8.3%; difference:2.5% (-0.1 to 5.2%), P=0.062]; the difference was greater at month 24 [9.3 versus 5.6%; difference: 3.7% (1.0 to 6.4%), P=0.007], at which time CLS was higher in the immediate arm (20.7 versus 15.7%, P=0.013). CLS-D-HIV risk at month 12 was substantially lower in the immediate versus deferred arm for MSM [0.2 versus 11%; difference:-10.7% (-12.5 to-8.9%), P<0.001] and heterosexuals [0.6% versus 7.7%; difference:-7.0% (-8.8 to-5.3%), P<0.001], because of viral suppression on ART.Conclusion:A strategy of early ART had no effect on condomless sex with HIV-serodifferent partners among MSM, but resulted in modestly higher prevalence among heterosexuals. However, among MSM and heterosexuals, early ART resulted in a substantial reduction in HIV-Transmission-risk sex, to a very low absolute level.