OBJECTIVE To assess the pharmacokinetics and tolerability of reduced dosages of twice daily indinavir (IDV) boosted by low-dose ritonavir (RTV) in healthy volunteers. METHODS Pharmacokinetics and tolerability of IDV/RTV twice daily (600/100 mg and 400/100 mg) were assessed in a randomized crossover design in 16 healthy volunteers. Each dosage was taken twice daily for 2 weeks before 12 h pharmacokinetics were obtained. RESULTS Sixteen subjects were included, with a mean age +/- SD of 30 +/- 4 years; seven female, nine male. Fifteen subjects completed the study. After dose reduction of IDV AUC, Cmax and Cmin decreased significantly. In the 400 mg group three out of 15 subjects had IDV levels below 0.10 mg/l vs none in the 600 mg group. All subjects reported mild to moderate side effects throughout the study period, which were more severe in the 600 mg group (mostly renal, dry skin/lips, paresthesias/oral discomfort). In the 600 mg group four subjects reported dysuria and one subject discontinued because of flank pain, whereas two subjects reported dysuria and no subject discontinued in the 400 mg group, respectively. Eight subjects developed crystalluria without a significant difference between both groups. No significant change in serum creatinine was observed. CONCLUSIONS IDV/RTV 400/100 mg twice daily resulted in significant lower IDV exposure, with three out of 15 subjects revealing Cmin values below the recommended threshold for wild-type virus of 0.10 mg/l. Tolerability, however, was lower in the 600 mg IDV group. Therapeutic drug monitoring in the individual patient appears to be necessary to guarantee appropriate drug levels and simultaneously minimize toxicity.