Ursodeoxycholic acid treatment (600 mg/day) was evaluated in a patient with asymptomatic primary sclerosing cholangitis. Serum levels of biliary enzymes decreased to normal ranges within 1 month's treatment and remained normal for 26 months. Serum chenodeoxycholic acid had been replaced by ursodeoxycholic acid, and hepatic copper metabolism, assessed by x-ray probe analysis, improved during the treatment. However, neither biliary tract sclerosis nor portal tract pathology changed with the treatment. These observations suggest that ursodeoxycholic acid protects the liver in primary sclerosing cholangitis by improving the metabolism of bile acid and copper.