A liver fibrosis index was recently prospectively validated in a cross-sectional study where patients infected by hepatitis C virus (HCV) had only one biopsy and no longitudinal follow-up. The aim of this study was to retrospectively assess the diagnostic value of this index in patients included in a randomized trial of interferon (IFN) using repeated measurements, two biopsies and hyaluronic acid as a comparative reference. One-hundred and sixty-five patients who had had two interpretable liver biopsies and at least one stored serum sample before IFN treatment were selected. Seventy-eight patients received 3 MU of IFN-alpha thrice weekly for 24 weeks and 87 followed a reinforced regimen for 48 weeks. A fibrosis index combining five biochemical markers (alpha2-macroglobulin, haptoglobin, apolipoprotein A1, gamma-glutamyl transpeptidase (GGT) and total bilirubin adjusted for gender and age) as well as hyaluronic acid was assessed on 461 samples available at baseline, at the end of treatment and at the end of follow-up (72 weeks). There was a significant decrease of the fibrosis index score among the 17 sustained virologic responders, from 0.33 +/- 0.06 (mean +/- SE) at baseline to 0.18 +/- 0.06 at 72 weeks in comparison with 92 nonresponders (from 0.41 +/- 0.03 at baseline to 0.44 +/- 0.03 at 72 weeks; P < 0.001) and in comparison with 56 relapsers (from 0.36 +/- 0.03 at baseline to 0.32 +/- 0.03 at 72 weeks; P=0.05). No significant differences were observed for hyaluronic acid.Hence, this fibrosis index could be used as a surrogate marker of the antifibrotic effect of treatments in patients with chronic hepatitis C.