Cohort studies have shown that highly active antiretroviral therapy (HAART) can improve liver-related mortality in HIV/hepatitis C virus (HCV)-coinfected patients. A reduction in the accelerated liver fibrosis progression observed in HIV infection induced by HAART could explain these findings. A few studies have assessed the impact of HAART on liver fibrosis, but with contradictory results. Therefore, we evaluated the associations between the use of different antiretroviral drug classes and HAART combinations, and liver fibrosis in HIV-infected patients with chronic hepatitis C. Six hundred and eighty-three HIV/HCV-coinfected patients, who underwent a liver biopsy and who had not received anti-HCV treatment were included. Age at HCV infection < 23years (adjusted odds ratio [AOR] = 0.7, 95% confidence interval [95% CI] = 0.3-0.9, P = 0.05) and protease inhibitor (PI)-based HAART versus no use of HAART (AOR = 0.5, 95% CI = 0.3-0.9, P = 0.01) were negatively associated with advanced fibrosis (> or = F3). PI-based HAART versus no use of HAART (AOR = 0.4, 95% CI = 0.2-0.7, P = 0.001) was negatively associated with fibrosis progression rate > or = 0.2 units/year and independently of age at HCV infection and CD4+ T-cell counts. Fifteen (17%) patients treated only with PIs and zidovudine plus lamivudine showed > or = F3, compared with 65 (37%) patients without HAART (P = 0.001). Forty (31%) patients on PI and stavudine plus lamivudine showed > or = F3 (P = 0.3, when compared with patients with no HAART). The use of PI-based HAART in HIV/HCV-coinfected patients is associated with less severe fibrosis and slower progression of fibrosis. The nucleoside analogue backbone in a HAART regimen may influence this association.