BACKGROUND Hepatitis C virus infection is more common in patients with inflammatory bowel disease than in general population. Limited data are available as to the safety and efficacy of alpha-interferon therapy for chronic active hepatitis C in patients with concomitant inflammatory bowel disease. AIM To evaluate the efficacy and safety of alpha-interferon monotherapy in patients with chronic active hepatitis C and inactive or mildly active inflammatory bowel disease. METHODS A total of 513 consecutive inflammatory bowel disease patients at a single centre were tested for antibodies to hepatitis C virus (anti-hepatitis C virus) between 1995 and 2000. Twenty-one patients had detectable anti-hepatitis C virus Ab and were hepatitis C virus-RNA positive with histologically proved chronic active hepatitis. Each of these patients, whose inflammatory bowel disease was in clinical remission or mildly active, was sex- and age-matched to three controls with similar histological grade and stage of chronic hepatitis C virus but without inflammatory bowel disease; and all were treated with human leucocyte alpha-interferon 6 million units given thrice weekly for 12 months. Responses to treatment were classified as follows: complete response--persistently normal alanine aminotransferase and viral clearance (hepatitis C virus-RNA-ve) at the end-of-treatment, incomplete response--alanine aminotransferase normalization without viral clearance (hepatitis C virus-RNA+ve), and sustained response--alanine aminotransferase normalization and hepatitis C virus clearance 12 months after the end-of-treatment. RESULTS Twenty-one patients with chronic active hepatitis C and inflammatory bowel disease (10 with Crohn's disease and 11 with ulcerative colitis) and 63 sex- and age-matched controls with chronic hepatitis C virus alone received alpha-interferon monotherapy. Response rates to interferon were similar for inflammatory bowel disease patients compared with controls [CR 42% vs. 35% and SR 24% vs. 18% (P, not significant), respectively]. None of the 21 inflammatory bowel disease patients had severe adverse effects and the mild ones observed were comparable with those seen in the control group. No patients developed an inflammatory bowel disease relapse during the interferon treatment period or in the 12 months thereafter. CONCLUSIONS The biochemical and virological response to a 12-month human leucocyte alpha-interferon treatment in patients with chronic active hepatitis C are similar to that observed in matched controls with chronic hepatitis C virus without inflammatory bowel disease. Adverse effects are similar in both groups of patients and unrelated to the underlying inflammatory bowel condition. This provides hepatologists with evidence that alpha-interferon can be safely administered to patients with chronic hepatitis C virus and inflammatory bowel disease provided that the inflammatory bowel condition is in clinical remission.