OBJECTIVE Our objective was to evaluate the steady-state pharmacokinetics of ritonavir-boosted atazanavir when coadministered with tenofovir in HIV-1-infected adult patients. METHODS Forty adult HIV-1-infected patients received either atazanavir/ritonavir 300/100 mg once daily and nucleoside reverse transcriptase inhibitors with (n = 20) or without (n = 20) tenofovir-disoproxil fumarate (tenofovir-DF) 300 mg once daily. Twenty-four-hour pharmacokinetics were assessed after at least 2 weeks of therapy according to a standardised therapeutic drug monitoring protocol. METHODS Atazanavir/ritonavir plasma concentrations were measured by liquid chromatography tandem mass spectrometry, and the geometric means of minimum and maximum concentrations (C(min), C(max)), the area under the time-concentration curve (AUC), half-life (t(1/2)) and total clearance (CL(tot)) were subject to a matched pairs-analysis. Patients' pairs were matched for gender, ethnicity, weight and Center for Disease Control and Prevention (CDC) status. RESULTS The respective geometric means (90% CI) for atazanavir C(min), C(max) and AUC with tenfovir vs. without tenofovir were 405 (314-523) vs. 417 (304-572) ng/ml, 3,022 (2,493-3,664) vs. 2,817 (2,341-3,390) ng/ml and 34,822 (29,315-41,363) vs. 32,101 (26,206-39,321) ng x h/ml showing no significant differences between the groups. Atazanavir plasma concentrations measured at week 5 of therapy or later were lower than in the first 4 weeks (T-test for C(max), p = .080; AUC, p = .050 and CL(tot), p = .051). CONCLUSIONS The coadministration of tenofovir-DF did not impair the plasma concentrations of ritonavir-boosted atazanavir in a pharmacokinetic analysis of patient pairs matched for gender, ethnicity, weight and CDC status.