BACKGROUND: Suboptimal levels of antiretroviral drugs result in virologic failure in HIV-infected patients treated with highly active antiretroviral therapy (HAART). OBJECTIVE: To assess the relationship between levels of indinavir in hair and virologic outcome. DESIGN: Cross-sectional study. SETTING: 7 AIDS clinics in France. PATIENTS: 89 HIV-infected patients who received HAART that included indinavir. MEASUREMENTS: Patients were classified as responders or nonresponders on the basis of viremia at the time of hair collection. In nonresponders, levels of indinavir in hair and resistance mutations in the protease gene were assessed at baseline and at the time of indinavir measurement. RESULTS: Mean indinavir levels (+/-SD) were significantly higher in the 65 responders than in the 24 nonresponders (24.4 +/- 16 microg/g vs. 12.9 +/- 8.6 microg/g) (P < 0.001). Nonresponders with intermediate levels of indinavir in hair had more mutations in the protease gene than did nonresponders with low levels of indinavir in hair. CONCLUSION: Indinavir levels in hair are associated with virologic outcome in patients receiving HAART.