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To estimate the effect of combined antiretroviral therapy (cART) on mortality among HIV-infected individuals after appropriate adjustment for time-varying confounding by indication.A collaboration of 12 prospective cohort studies from Europe and the United States (the HIV-CAUSAL Collaboration) that includes 62 760 HIV-infected, therapy-naive individuals followed for an average of 3.3 years. Inverse probability weighting of marginal structural models was used to adjust for measured confounding by indication.Two thousand and thirty-nine individuals died during the follow-up. The mortality hazard ratio was 0.48 (95% confidence interval 0.41–0.57) for cART initiation versus no initiation. In analyses stratified by CD4 cell count at baseline, the corresponding hazard ratios were 0.29 (0.22–0.37) for less than 100 cells/μl, 0.33 (0.25–0.44) for 100 to less than 200 cells/μl, 0.38 (0.28–0.52) for 200 to less than 350 cells/μl, 0.55 (0.41–0.74) for 350 to less than 500 cells/μl, and 0.77 (0.58–1.01) for 500 cells/μl or more. The estimated hazard ratio varied with years since initiation of cART from 0.57 (0.49–0.67) for less than 1 year since initiation to 0.21 (0.14–0.31) for 5 years or more (P value for trend <0.001).We estimated that cART halved the average mortality rate in HIV-infected individuals. The mortality reduction was greater in those with worse prognosis at the start of follow-up.