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Objectives (1) To document rates and patterns of adherence from enrollment until week 24 of an AIDS clinical trial; (2) to examine the association of adherence to clinical end-points including plasma HIV-1 RNA level and CD4 cell count; and (3) to identify predictors of adherence from clinical, behavioural, psychosocial and demographic factors.Design Sub-study of a multicentre, randomised, open-label, comparison-controlled trial; 21 collaborating units of the Adult AIDS Clinical Trials Group. Observational, prospective analysis.Methods Ninety-three subjects with baseline plasma HIV-1 RNA levels >500 copies/ml, who completed clinical assessment, plasma HIV-1 RNA titres and CD4 cell counts at study entry, weeks 2, 4 and every 4 weeks thereafter until week 24. All patients were antiretroviral-experienced but were naive to non-nucleoside reverse transcriptase inhibitors and protease inhibitors. Self-reported adherence to antiretroviral therapies prescribed as part of the trial was assessed every 4 weeks from trial, week 4 until week 24.Results Average adherence was high, with 63% of subjects reporting >95% adherence across the trial. However, there was a significant decline in adherence over time on trial. After controlling for potential confounding variables, patients who were less than 95% adherent to medications were 3.5-times more likely to have treatment failure (HIV-1 RNA >50 copies/ml) than subjects with adherence rates of 95–100%. The strongest predictor of adherence was adverse clinical events (for example, dermatological, gastrointestinal symptoms): patients with adverse events were 12.8-times less likely to have 95–100% adherence. Other clinical, demographic, psychosocial and behavioural factors were also significant predictors of adherence.Conclusions Adherence influences virological outcome even in AIDS clinical trials where overall adherence rates are high and should therefore be monitored in future trials. Intervention may be warranted to enhance adherence for subjects who have early toxicities, express concern about taking medications as directed, and for women and minorities.