The potent induction of hepatic cytochrome P450 3A isoforms by rifampin complicates therapy for coinfection with human immunodeficiency virus (HIV) and Mycobacterium tuberculosis. We performed an open-label, single-arm study to assess the safety and pharmacokinetic interactions of the HIV protease inhibitor atazanavir coadministered with rifampin. Ten healthy HIV-negative subjects completed pharmacokinetic sampling at steady state while receiving 300 mg atazanavir every 12 h without rifampin (period 1), 300 mg atazanavir every 12 h with 600 mg rifampin every 24 h (period 2), and 400 mg atazanavir every 12 h with 600 mg rifampin every 24 h (period 3). During period 1, the mean concentration of drug in serum at 12 h (C(12 h)) was 811 ng/ml (range, 363 to 2,484 ng/ml) for atazanavir, similar to historic seronegative data for once-daily treatment with 300 mg atazanavir boosted with 100 mg ritonavir. During periods 2 and 3, the mean C(12 h) values for atazanavir were 44 ng/ml (range, <25 to 187 ng/ml) and 113 ng/ml (range, 39 to 260 ng/ml), respectively, well below historic seronegative data for once-daily treatment with 400 mg atazanavir without ritonavir. Although safe and generally well tolerated, 300 mg or 400 mg atazanavir administered every 12 h did not maintain adequate plasma exposure when coadministered with rifampin.