To assess the factors associated with liver fibrosis in human immunodeficiency virus and hepatitis C virus (HIV/HCV) co-infected patients eligible for anti-HCV therapy, we performed an observational, single-centred, cross-sectional study of 180 HIV/HCV co-infected patients who underwent liver biopsy between May 1998 and November 2001. A total of 126 patients with a known date of HCV infection were evaluated. Liver fibrosis was defined as a Knodell stage of fibrosis 1-4. The mean age was 36.7 (3.8) years, 81% were male and had a mean age of 20.5 (3.8) years at HCV infection. Mean CD4 cell count and plasma HIV-1 RNA load at the time of biopsy were 552 cell/mm3 (239) and 2.5 log10 (0.9), respectively; 118 patients had been on antiretroviral therapy (ART) for a median of 45 months (Q1-Q3: 21-75) and 84 on protease inhibitor for a median of 12.0 months (Q1-Q3: 0-29.5); 55 had an AIDS event or a CD4 cell count nadir < 200 cells/mm3 prior to biopsy. Median histological activity index was 6 and 27% had a Knodell stage of fibrosis 0. On the multivariate analysis time on ART (OR for 6 months extra: 0.954, 95% CI: 0.859-0.994), CD4 cell count at the time of liver biopsy (OR for 100 cells/mL increase: 0.740, 95% CI: 0.670-0.905), age at HCV infection acquisition (OR for 5 years extra: 2.594, 95% CI: 1.326-5.133) and alcohol intake (> 50 g/day) (OR: 2.73, 95% CI: 1.108-6.731) were associated with liver fibrosis. Hence ART should be a priority in HIV/HCV co-infected patients eligible for anti-HCV treatment as it is a protective factor for liver fibrosis.