Although autoantibodies have been found in the serum of patients with chronic hepatitis C virus (HCV) there has been no convincing evidence of the presence of antimitochondrial antibodies, until now. Sera from 460 untreated patients with chronic hepatitis C were tested for antimitochondrial antibodies, using an indirect immunofluorescence technique; and if they tested positive for the antibodies (titer more than 1:50), they also were treated by Western blot analysis. Seven (1.5%) sera were positive. None of the patients had biological or histological evidence of primary biliary cirrhosis. Antimitochondrial antibodies recognized one of the oxo-dehydrogenase multienzyme complexe's epitopes by Western blot assay in three patients only. All seven patients were then treated by interferon alpha for six months. None showed exacerbation of liver disease during treatment. HCV-RNA disappeared from the serum in one patient who became negative for anti-M2 antibodies. The four patients who did not respond to interferon-alpha therapy, and the two who relapsed after treatment withdrawal, had sustained positive antimitochondrial antibodies. These data suggest that: 1) antimitochondrial antibodies present in patients with chronic hepatitis C do not always recognize the same epitopes as in primary biliary cirrhosis; 2) these antibodies may disappear after eradication of HCV, suggesting that the production of antimitochondrial antibodies is linked to the presence of the virus and 3) the clinical and biological course of chronic hepatitis C, and the response to interferon-alpha therapy, does not seem to be different in patients who are positive for antimitochondrial antibodies.