A reduction in serum enzymes has been already observed by administering ursodeoxycholic acid to patients with chronic active hepatitis. The aim of this study was to assess whether the Liver histological activity of inflammation was modified by a 12-month treatment with ursodeoxycholic acid. Thirty-six patients with chronic active hepatitis, fulfilling the inclusion criteria, were admitted to the trial. Patients were randomly allocated to receive double blind either 600 mg/day of ursodeoxycholic acid (Group A: 18 patients) or placebo (Group B: 18 patients). Clinical and biochemical follow-up was performed at 3-month intervals. A percutaneous liver biopsy was performed before and after 1 year of treatment. Histological hepatitis activity was assessed using Knodell's numerical scoring system, while biliary damage was evaluated by an appropriate scoring system. Sixteen and 12 patients in Groups A and B, respectively, completed the clinical and biochemical follow-up. Although a reduction in serum enzymes was found in both groups, multifactorial covariance analysis showed that the reductions in alanine aminotransferase, aspartate aminotransferase and gamma glutamyl transpeptidase were significantly higher in Group A than in Group B. Biochemical remission was not observed in either group. Histological analysis showed a dichotomy between the results from the hepatitis and the biliary components of the disease process. No differences were found in the two groups before or after treatment in histological activity index, which measures the ''hepatitic'' component. Nor were there any significant differences in baseline values. Furthermore, no relationship was found between changes in biochemical parameters and the histological activity indices. In contrast, the ''biliary'' component showed significant improvement after ursodeoxycholic acid treatment for certain cholestatic parameters: ductular metaplasia, bile duct damage and phenotypic cytokeratin changes. These changes were significantly associated with changes in gamma glutamyl transpeptidase serum levels. This study has confirmed that ursodeoxycholic acid administration in patients with chronic active hepatitis reduces transaminases and gamma glutamyl transpeptidase. The morphological substrate of this biochemical improvement can be traced to the biliary component of the process. (C) Journal of Hepatology.