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To describe the pharmacokinetics of atazanavir (ATV) and ritonavir-boosted ATV (ATV/r) in children aged 91 days to 21 years.A phase I/II, open-label, multicenter study of once-daily ATV and ATV/r as part of combination antiretroviral treatment in HIV-infected treatment-experienced and treatment-naive children.Sites in the United States and South Africa.One hundred and ninety-five children enrolled; 172 had evaluable ATV pharmacokinetics on day 7.Children were entered in age, dose, and formulation (powder or capsule) cohorts. Intensive pharmacokinetic sampling occurred 7 days after starting ATV. ATV doses were increased or decreased if the 24-h area under the concentration time curves (AUC0–24hr) were less than 30 or more than 90 μg × h/ml, respectively.Cohorts satisfied protocol-defined pharmacokinetic criteria if the median ATV AUC0–24hr was 60 μg × h/ml or less, and AUC0–24hr and ATV concentrations 24-h postdose (C24) were more than 30 μg × h/ml and at least 60 ng/ml, respectively, in at least 80% of the children, with no individual AUC0–24hr less than 15 μg × h/ml.Unboosted ATV capsules satisfied pharmacokinetic criteria at a dose of 520 mg/m2 for those aged more than 2 to 13 years or less and 620 mg/m2 for those aged more than 13 to 21 years or less. ATV/r capsules satisfied criteria at a dose of 205 mg/m2 for those aged more than 2 to 21 years or less. ATV/r powder satisfied criteria at a dose of 310 mg/m2 for those aged more than 2 to 13 years or less, but pharmacokinetics in those aged 2 years or less were highly variable.Body surface area-determined doses of ATV capsules and of ATV/r powder and capsules provide ATV exposures in children of more than 2 years that approximate concentrations in adults receiving ATV/r.