To determine the incidence of hepatitis C virus (HCV) infection in patients with alcoholic liver disease (ALD), serum samples from 252 patients with ALD were tested for anti-HCV and HCV RNA. Serial sera of these patients were collected and stored under optimal conditions to allow exact quantification of HCV RNA. Fifteen patients who visited our hospital during the same period of time with chronic HCV infections served as controls. In those with ALD, anti-HCV and HCV RNA were positive in 55.5% and 41.2%, respectively. Patients with histologically diagnosed chronic hepatitis and hepatocellular carcinoma had much higher prevalence rates of HCV RNA (84% and 100%, respectively) compared to those with fatty liver (4.3%), hepatic fibrosis (10.1%) and alcoholic hepatitis (22.2%) (P < 0.01). Although no difference in serum HCV RNA levels was observed between the patients with both ALD and chronic HCV infection and those with chronic HCV infection alone, HCV RNA levels significantly (10-fold) dropped after abstinence in nearly half of the patients (P < 0.01). These data indicate that HCV infection in patients with ALD promotes progression of liver disease, and abstinence from alcohol is associated with a reduction in serum HCV RNA levels.