In both adults and children, the increased risk of tuberculosis as a result of advancing human immunodeficiency virus (HIV) infection is a major contributor to HIV-associated morbidity and mortality. This is especially so in African settings, where the background risk of tuberculosis is already high. It is predictable that anti-retroviral therapy (ART), through allowing recovery of immune function, should reduce this increased risk in patients with HIV, and a number of studies in adults have indeed demonstrated such reductions. 1–3 Quantifying this reduction is complicated by the fact that the same clinical parameters that determine tuberculosis risk are also determinants of who starts ART and are themselves affected by ART, leading to potential time-varying confounding. Statistical methods appropriate to causal questions of this nature are increasingly being used in observational clinical studies. 4 In this issue, Edmonds and colleagues describe the impact of ART on tuberculosis risk in a cohort of HIV-infected children, based on a Cox proportional hazards marginal structural model. 5 The only other quantification of tuberculosis risk reduction due to ART that has, to date, employed such an approach was a study in South African adults, in which the risk reduction due to ART was 39%. 6 In the present study, the equivalent estimate is 49%. These estimates are much lower than many would have anticipated and than some of the prior estimates in adults and children. A recent study in HIV-infected South Africa children, which did not adjust for time-varying confounding, estimated the risk reduction on ART to be 70% for all tuberculosis cases as well as for microbio