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Pediatric experience with atazanavir combined with antiretroviral drugs administered once daily is very limited.The objective of this prospective, single-center observation study was to evaluate efficacy and tolerance of once-a-day ritonavir-boosted atazanavir, including treatment.Antiretroviral treatment of 23 children and adolescents with a median age of 16 years (range, 10–19 years) was changed to a single daily dose of a combination of ritonavir-boosted atazanavir and 2 other nucleoside or nonnucleoside analogs. The single daily dosing was expected to improve adherence to treatment. The mean follow-up period was 12 months (range, 6–17 months). At the time of the treatment switch, the previous treatment had been effective in 11 children (plasma viral load [pVL] <50 copies/mL) and not effective in 12 (pVL >50 copies/mL). None of the viral genotypes had resistance to atazanavir. The susceptibility score for the drugs used in combination with atazanavir (GSS) was at least 1.5 in 12 of 20 children. The atazanavir dose was 300 mg per day for children weighing more than 50 kg and 200 mg per day for children weighing 30 to 50 kg, in all cases associated with 100 mg ritonavir. During follow up, the mean atazanavir plasma concentration at 12 to 15 hours was 2.18 ± 1.19 mg/L. Tolerance was good in most patients, but 4 children chose to stop treatment because of icterus (n = 2) or persistent nausea and vomiting (n = 2). In 6 of the 12 children in whom treatment was not virologically effective before the switch, pVL was below 50 copies/mL after 1 to 3 months of treatment. Poor compliance and virologic failure persisted in the other 6 children. Seven of the 11 children with good virologic control before the switch continued to have undetectable pVL but 4 experienced virologic failure after 1, 1, 3 or 12 months of treatment despite good compliance. Insufficient antiviral potency of associated drugs could have been the cause of 2 of these 4 unexpected virologic failures.In these children with extensive previous treatment, the change to a once-daily treatment, including ritonavir-boosted atazanavir, was associated with a significant risk of virologic failure.