Human immunodeficiency virus (HIV) patients on nucleoside or nucleotide reverse transcriptase inhibitors with HIV RNA at <1,000 copies/ml were randomized in an open-label study to administration of combined indinavir/ritonavir (IDV/RTV) at 667/100 mg every 12 h (q12h) or IDV alone at 800 mg q8h to determine the regimens' pharmacokinetics. On day 14, plasma IDV and RTV levels were determined over 24 h. Noncompartmental pharmacokinetics (minimum concentration of drug in serum [Cmin], area under the concentration-time curve from 0 to 24 h [AUC0-24], and maximum concentration of drug in serum [Cmax]) were expressed as geometric mean values with 90% confidence intervals (CI). The primary hypothesis was that the lower bound of the protocol-specified 90% CI for the geometric mean C min ratio of the combination compared to IDV alone regimen would be ≥2. Twenty-seven patients were enrolled, and 24 (15 male; average age, 42 years) completed the study. The Cmin, AUC0-24, and C max for IDV/RTV compared to IDV alone were 1,511 versus 250 nM, 119,557 versus 77,034 nM · h, and 10,428 versus 10,407 nM, respectively. Corresponding relationships for IDV/RTV compared to IDV alone were a 6.0-fold increase in Cmin (90% CI, 4.0, 9.3), an increase in AUC 0-24 (1.5-fold, 90% CI, 1.2, 2.0), and no increase in C max. Adverse evems events were similar and generally mild, with no cases of nephrolithiasis. The geometric mean ratio of IDV Cmin for IDV/RTV compared to IDV was at least 2 by a lower bound of the 90% CI, satisfying the primary hypothesis. The Cmax was not increased, suggesting an IDV/RTV 667/100-mg toxicity profile may be similar to that of unboosted IDV.