Indinavir is widely prescribed as a component of potent antiretroviral therapy for the treatment of HIV-1 infection. Because virologic failure of therapy can result from subtherapeutic drug levels, monitoring of indinavir levels may be important in clinical management. We have developed a simple, accurate, and precise high-performance liquid chromatographic (HPLC) assay for measurement of indinavir concentration in human plasma. In our method, indinavir was extracted from plasma with dichloromethane at pH 10.4, which resulted in quantitative recovery of indinavir and the internal standard (IS), methyl-indinavir (86% and 80%-97%, respectively). Chromatographic separation was accomplished using a Luna C18 (2) (Phenomenex) analytic column with a mobile phase composed of acetonitrile:phosphate buffer (25 mM) and 0.2% triethylamine pH 7.0 (34.5:65.5, v/v). Ion-paired reagent triethylamine was necessary to ensure an appropriate retention time for indinavir and differentiate it from other protease inhibitors that were coextracted. Quantification was performed at 210 nm. The standard curves were linear (r2>0.999) over the concentration range 25-5,000 ng/mL, when 1-mL aliquots of plasma were extracted. Inter- and intraday coefficients of variation were acceptable. The assay was used to determine trough and peak levels of in plasma from 12 subjects who received indinavir 1200 mg every 12 hours, 1000 mg every 12 hours, or 800 mg every 8 hours. The concentrations of indinavir found in this study (trough 26-768 ng/mL; peak at 1 hr 3,309-17,568 ng/mL) has a wider range than defined previously (trough 50-300 ng/mL; peak 6,000-12,000 ng/mL). This study illustrates three potential uses of indinavir monitoring: to assess individual dosing regimen, to assess patient compliance, and to monitor unusual indinavir levels caused by changed drug clearance.