Ursodeoxycholic acid (UDCA) has been reported to improve liver function tests when administered to patients with cholestatic liver diseases, such as primary biliary cirrhosis (PBC). However, its effects on biliary lipid metabolism in patients with PBC are still unknown. In this study we report the effect that UDCA (600 mg/day, for four weeks) had on biliary cholesterol saturation index, biliary bile acid pattern and pool size, and biliary lipid output in seven female patients (ages 34-58 years) with PBC, stages I to III. A significant improvement of liver function tests was observed after four weeks of treatment. Saturation index was significantly decreased from 1.23 +/- 0.1 to 0.7 +/- 0.08 (P less than 0.02); this effect was due to the significant decrease of biliary cholesterol concentration from 6.7 +/- 0.36 to 3.6 +/- 0.37 percent molar (P less than 0.02). A significant decrease of cholesterol output (from 88 +/- 9 to 55 +/- 10 mumol/hr, P less than 0.02) was also observed. The amount of cholic acid, the predominant bile acid in bile, significantly decreased (from 47.3 +/- 3.5 to 35.4 +/- 2.6 percent molar, P less than 0.02), as did amounts of chenodeoxycholic and deoxycholic acids, while the amount of UDCA rose from 1.6 +/- 1.0 to 34.0 +/- 1.3 percent molar (P less than 0.02). Total bile acid pool size was not affected by UDCA, but the evaluation of individual bile acid pool sizes showed an increased proportion of UDCA relative to the endogenous bile acids.(ABSTRACT TRUNCATED AT 250 WORDS)