In a total of 1004 patients in 11 controlled trials, treatment with ursodeoxycholic acid (UDCA) 8-15 mg/kg bodyweight per day led to a decrease of pruritus in 30-60% of cases, a decrease in aminotransferases and cholestasis-indicating enzymes in serum by 20-80%, and a decrease of serum bilirubin by 3-40%. A statistically significant improvement in liver histology was found in only two of these studies; in three others there was a positive trend. In three more trials histology was not examined, and in three studies there was no improvement. In the four studies investigating the time elapsed before liver transplantation and the number of deaths, only one definitely found that this was prolonged by UDCA, although in two of the other three there was a positive trend. During treatment, UDCA constitutes 30-50% of the total bile acids in bile and serum; however, its influence on the toxic bile acids is debatable. Cholic acid decreases, but deoxycholic acid and chenodeoxycholic acid are reduced to a lesser degree. UDCA therapy has now been practiced for 12 years and all authors consider the treatment to be safe, but the mode of action of UDCA is still unknown.