One hundred and twenty-seven patients with histologically verified chronic non-A, non-B hepatitis were followed for up to 23 years (mean 6.3 years). Thirty-nine were infected by blood transfusion, 58 were drug-addicts and 30 had no obvious source of infection. Chronic persistent hepatitis (CPH) was diagnosed in 84 (66%), while 43 patients (34%) had chronic active hepatitis (CAH) with or without cirrhosis. Patients with CPH were significantly younger (29.7 years vs 46.8 years; p less than 0.01), irrespective of the type of virus exposure. Antibodies to hepatitis C virus (anti-HCV) were detectable in 91 patients (72%) and 36 (28%) were anti-HCV negative. Fifteen patients with acute onset, and negative for anti-HCV at the start, became positive during follow-up; 12 of them within 4.5 months. We found no differences among anti-HCV positive and anti-HCV negative patients in liver function tests, resolving rate, morphological progression in serial biopsies or mortality rate. Five previously anti-HCV positive patients became negative during follow-up and in two of them this was accompanied by decreasing hepatic inflammation.