We investigated the effects of once-daily oral administration of 10 mg/kg ursodeoxycholic acid (generic name, ursodiol) on elevated serum enzyme activities, bilirubin, cholesterol, bile acids and symptoms in patients with primary sclerosing cholangitis. A 30-mo, open-label, pilot trial was designed to cover four periods: (a) 3 mo of pretreatment observation (period 1), (b) 6 mo on ursodiol (period 2), (c) 3 mo withdrawal of treatment (period 3) and (d) 18 mo of extended retreatment (period 4). Diagnosis was confirmed by cholangiography and liver biopsy specimens. We enrolled 12 patients with persistently elevated pretreatment alkaline phosphatase and gamma-glutamyltransferase levels (at least twice the upper limit of normal), and observed them for a median of 37 mo. Significant reductions in serum total cholesterol levels and in serum enzyme activities indicating cholestasis and hepatocellular injury occurred during ursodiol treatment in both treatment periods 2 and 4 and relapsed with treatment interruption in period 3. Elevated serum bilirubin and symptoms of disabling fatigue, pruritus and diarrhea were improved by ursodiol. Improvements have continued after 2 yr of treatment in 10 patients (1 patient had a transplantation after he relapsed on withdrawal of ursodiol therapy; another died of postoperative complications of colon resection for carcinoma). No other cases of clinical deterioration were observed in the retreatment period. The longer term reductions of alkaline phosphatase, transaminases, bilirubin and cholesterol after 2 yr of treatment were even greater than the initial reductions after 6 mo of treatment. These results justify initiation of larger, controlled clinical trials, with serial morphological evaluations of the liver and biliary tree.