The aim of this study was to assess the causes of histologically proven chronic hepatitis in a series of 357 consecutively admitted patients. Patients with chronic alcohol intake above 50 g per day, Wilson's disease, idiopathic hemochromatosis or homozygous alpha-1 antitrypsin deficiency were excluded. Sera of all patients were tested for antibodies to hepatitis C virus with second-generation enzyme-linked immunoassay and recombinant immunoblot assay, for markers of hepatitis B and hepatitis D viruses, and for autoantibodies. Detection of hepatitis C viral RNA by polymerase chain reaction was attempted if recombinant immunoblot assay was indeterminate, or if both viral and autoimmune markers were absent. If no serum markers, including HCV RNA, were found, the cause of chronic hepatitis was considered as unknown. The cause of chronic hepatitis was found in 343 cases (96.4%), including three patients with HCV RNA as the only marker. Chronic hepatitis was related to hepatitis C virus in 51.8%, to hepatitis B virus in 32.8% (including hepatitis D infection in 3.1%), and to autoimmune hepatitis in 5.9% of cases, respectively. No case of drug-induced chronic hepatitis was observed in this series, and in 5.9% of cases, there were probably multiple causes. Finally, in 3.6% of the cases the cause of chronic hepatitis remained unknown despite extensive evaluation suggesting the existence of a non-A, non-B, non-C viral agent.