Because 70% to 75% of patients with chronic hepatitis C either do not respond to or relapse after interferon (IFN) therapy, and because ursodeoxycholic acid (UDCA) has been shown to reduce aminotransferase levels in patients with chronic hepatitis, we undertook a prospective controlled randomized trial of IFN (group I) versus IFN plus UDCA (group II) in 80 patients with chronic hepatitis C. IFN was administered in both groups for 6 months (3 to 5 million units [MU] three times a week), and in group II UDCA (10 mg/kg/d) was administered with IFN and then alone for 3 additional months. Response to therapy was defined as the normalization of alanine transaminase (ALT) levels. The results showed that 6 months after cessation of IFN, 59% of responders had relapsed in group I but only 27% had relapsed in group II (P = .03). There was no difference between the two groups for the initial (month 6) and the late (months 15 and 18) response rates to IFN. There was no virological effect or significant histological improvement attributable to the addition of UDCA to IFN treatment. In conclusion, the results of this study show that the addition of UDCA to IFN therapy significantly prolongs the period for which serum ALT remain, within the normal range after discontinuation of IFN. Further studies would be required to determine whether UDCA has any potential for long-term amelioration of the histological severity of liver disease caused by hepatitis C virus (HCV) infection, and, therefore, whether it could be advocated as an adjunct to antiviral therapy.