We explore changes in sexual risk behaviour over the first year of antiretroviral therapy (ART) among a cohort of patients in Cape Town South Africa initiating treatment in five public facilities in 2006 and again 1 year later (Time 1 and Time 2). Contemporaneous measures of unprotected sex were also obtained from 2 cross-sectional samples of HIV-positive patients waiting to start ART attending the same facilities. Unprotected sex at last sex among patients on ART decreased significantly from a baseline of 44.7-23.2% one year later, regardless of partner status. After controlling for confounding factors, the observed decrease in unprotected sex among the ART cohort was highly significant in relation to the 2 cross-sectional samples of patients at Time 1 and Time 2 waiting to initiate ART. Findings suggest it is critical to start positive prevention to decrease risky sexual behavior prior to the start of ART within this setting.