Ursodeoxycholic acid (UDCA) has recently been combined with interferon (IFN) in the treatment of individuals with chronic hepatitis C. However, whether its addition results in a long-term favourable response to IFN remains unclear. A prospective randomized trial of IFN alone versus IFN plus UDCA was therefore undertaken in 52 patients with chronic hepatitis C. All patients received a 24 week course of IFN-alpha (6 x 10(6) U/day for 2 weeks and then three times a week for 22 weeks) and half also received UDCA (600 mg/day) with IFN and then alone for 48 additional weeks. Normalization of serum alanine transaminase (ALT) concentrations at 0, 24 and 48 weeks after cessation of IFN therapy was apparent in 77, 42 and 42% of patients in the IFN-alone group and in 77, 54 and 42% of patients in the IFN plus UDCA group, respectively. There was no significant difference between the two groups with regard to response rate to IFN and the addition of UDCA to IFN treatment had no significant effect on hepatitis C virus (HCV) viraemia. During the follow-up period, 10 of 20 patients with normal serum ALT at the end of IFN treatment relapsed in the IFN-alone group compared with 11 of 20 patients in the IFN plus UDCA group. Among these relapsed patients, serum ALT concentration was significantly lower in the IFN plus UDCA group than in the IFN-alone group during the follow-up period. Twenty-four weeks after cessation of IFN therapy, the percentage of patients with HCV-RNA in their serum who showed a normalization of serum ALT concentrations was significantly higher in the IFN plus UDCA group than in the IFN-alone group (44 vs 6%). Thus, although the addition of UDCA was not associated with a favourable long-term response to HCV viraemia, it did reduce the risk and the severity of relapse following the cessation of IFN therapy.