Alcohol changes the progression of hepatitis C virus (HCV)-related chronic liver disease and may affect the outcome of interferon therapy. The ethanol intake of 245 patients with biopsy-proven chronic hepatitis C with or without cirrhosis, its interaction with laboratory and histological parameters common to alcohol and HCV-mediated liver damage, and its effects on therapy were evaluated. The results show that 60–70% of subjects regularly consumed alcohol (median intake >40 g/day in about 30%). Less than 50% stopped drinking after being diagnosed as having liver disease. Ethanol intake affected: fibrosis, especially in women, HCV RNA levels, which were significantly lower in abstainers than in drinkers (0.6 ± 0.3 vs 6.9 ± 5.9 Eq/ml x106; P < 0.01), and response to interferon therapy. The number of responders decreased as ethanol intake increased. There were less abstainers than drinkers among non-responders (10.7% vs 63.1% respectively; P < 0.001). Data indicate that alcohol will induce and worsen liver damage and, in subjects with chronic liver disease who continue to drink, adversely affect their response to treatment.