A retrospective multicentre survey was conducted to evaluate, in patients with chronic hepatitis C, the long-term liver histological changes induced by interferon (IFN). A total of 112 patients (mean age 46.4 years) were studied. All patients had received a 6-12-month IFN-alpha course (6-18 MU/week) and had successively undergone clinical, biochemical and virological follow-up for at least 36 months (range: 36-76). In each patient, two liver biopsies had been performed: 1-6 months before treatment and, 12-76 months after its completion. In 87 patients with biochemical and virological sustained response persisting for 12 months after therapy, post-treatment liver necroinflammation and fibrosis mean(+/-SD) scores (Knodell index) were significantly lower than pretreatment scores (2.9 +/- 2.2 vs 6.8 +/- 2.9 and 0.8 +/- 1.0 vs 1.2 +/- 1.1, respectively; P < 0.01). In 25 patients who relapsed within 1 year, necroinflammation and fibrosis post-treatment mean scores were similar to pretreatment scores (7.4 +/- 3.2 vs 6.9 +/- 3.1 and 1.8 +/- 1.3 vs 1.6 +/- 1.2, respectively; P > 0.05). On an individual basis, necroinflammation decreased in 87% of sustained responders but only in 36% of relapsers (P < 0.001), whereas fibrosis decreased in 44% of sustained responders but only in 14% of relapsers (P < 0.001). In sustained responders with biopsies performed 12-23 months (n=34), 24-35 months (n=26) or more than 36 months (n=27) after treatment, a progressive decrease of mean necroinflammatory score was observed (-2.6 +/- 2.1, -4.1 +/- 3.4 and -5.2 +/- 3.7 points, respectively; P < 0.01). A similar pattern was observed in fibrosis score (-0.3 +/- 0.6, -0.3 +/- 0.7 and -0.7 +/- 0.9 points, respectively; P < 0.05). Hence, among chronic hepatitis C patients treated with IFN, those with a 12-month sustained response, unlike those who relapse, have a long-term progressive reduction and, in some cases, a complete regression of liver histological damage.