Antiretroviral therapy (ART) can minimize HIV transmission. Prevention benefits may be compromised by barriers to virologic suppression, and by increased condomless sex among those initiating ART. We evaluated condomless sex in a cohort of HIVinfected US individuals poised to initiate ART in a clinical trial. We assessed partner and sex act type, condom use, and perception of infectiousness. Six percent of participants reported as not infectious; men who have sex with men were more likely to perceive high infectivity. Prevalence of condomless sex was 44 %; 74 % of those also reported homosexual acquisition of HIV. Predictors of increased risk of condomless sex included greater numbers of lifetime partners, recent stimulant drug use and an HIV-positive or unknown serostatus partner. In the context of serodifferent partners, lower perception of infectiousness was also associated with a higher risk of condomless sex. Results highlight opportunities for prevention education for HIV infected individuals at ART initiation.