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There have been rapid and dramatic increases in the numbers of people on antiretroviral treatment in Africa . These numbers should continue to grow substantially, if current treatment need is met and those progressing to later stages of infection are added into treatment programmes, especially if earlier initiation on treatment is adopted. Antiretroviral treatment itself may suppress HIV transmission rates by reducing viral load [2,3], but its preventive effect in populations will depend on its impact on sexual behaviour, with increases in risky behaviour potentially negating the benefits of reduced transmissibility and decreases in risky behaviour enhancing them. Indeed, a substantial part of the modelled impact of the ‘test and treat’ strategy promoted by the WHO was derived from an assumed change in risky behaviour . The paper by Venkatesh et al. published in this issue of AIDS suggests that initiation on treatment could lead to substantial reductions in risk behaviour in urban and rural populations in South Africa, which, if they can be replicated when treatment is initiated earlier and can be sustained for long periods after patients have regained their full health, would provide dramatic prevention benefits. These findings are very encouraging, especially because the comparison group comprised infected individuals who had previously received testing and counselling and could, therefore, already have adopted safer behaviour . However, before we conclude that treatment will enhance HIV prevention efforts by reducing risk behaviour, we should consider, first, whether such a big effect in antiretroviral therapy (ART) patients is plausible – or is more likely to have resulted from methodological difficulties – and, second, what the influence of ART might be on the behaviour of susceptible and undiagnosed infected individuals.The study by Venkatesh et al. has a prospective design (spanning periods up to and after ART initiation), a large representative sample (n = 6263 of whom 37% initiated treatment) with regular follow-ups, collected data on both sexes in both urban and rural areas, and applied rigorous statistical methods. As in some previous studies in sub-Saharan Africa [7,8], initiation on ART was associated with reductions in sexual activity, including reporting of unprotected sex and multiple sexual partnerships. However, the effect sizes in this study were particularly remarkable. The authors did not investigate the reasons for the reductions in risk behaviour in their setting and we wonder about the plausibility of such big effect sizes. Why would people who had been seriously ill (WHO stage 4 disease and/or CD4 cell count < 200 cells/μl.) and are now returned to good health become less sexually active and have fewer partners? Counselling and condoms were provided to newly diagnosed individuals as well as to those initiated on treatment; if disclosure to partners or fear of infecting others increased adoption of safer behaviours why would this be different for those who know they are infected, but are not yet treated? Methodological difficulties remain a concern: if interviewers were not blind to who was on treatment, interviewer bias could have affected the results. We are told that interviewers were trained to avoid social desirability bias, but this is very difficult to achieve especially when interviews are conducted in a clinical setting. It must be possible, therefore, that participants on ART felt a greater need to conceal risky behaviour. Concern over validity would be much assuaged by a good explanation for the behaviour change. In addition, trials showing the impact of expanded treatment on HIV incidence in populations would be welcome.