Objectives A proportion of HIV-positive people have condomless sex. Antiretroviral treatment (ART) reduces infectiousness, but a substantial proportion of HIV-diagnosed people are not yet on ART. We describe baseline self-reported risk behaviours in ART-naive Strategic Timing of AntiRetroviral Treatment (START) trial participants. Methods All START participants completed a risk behaviour questionnaire. Data were collected on sociodemographics, lifestyle factors, health and wellbeing status and clinical status. Recent sexual behaviour and HIV transmission beliefs in the context of ART were also assessed. The primary interest was in condomless sex with serodifferent partners (CLS-D) in the past two months. Results A total of 4601 of 4685 HIV-positive participants (98%) completed the questionnaire [2559 men who have sex with men (MSM), 803 heterosexual men and 1239 women]. Region of recruitment was Europe/Israel, 33%; South America/Mexico, 25%; Africa, 22%; other, 21%. Median age was 36 years [interquartile range (IQR) 29, 44 years]. Forty-five per cent reported white ethnicity and 31% black ethnicity. Two per cent had HIV viral load