The effect of ursodeoxycholic acid on recurrence prevention of choledochal calculi after endoscopic sphincterotomy (EPT) was evaluated in 46 patients, whose bile duct stones had been removed endoscopically. 22 patients received 500 mg of ursodeoxycholic acid once a day, 24 patients received placebo. 1 recurrent stone could be detected in the ursodeoxycholic acid treated group 19 months after EPT, whereas 4 recurrent stones occurred in the placebo group about 16 months after EPT. Cholesterol saturation index decreased significantly by the ursodeoxycholic acid treatment 12 and 24 months after EPT (0.93 +/- 0.17 vs. 0.69 +/- 0.12 vs. 0.72 +/- 0.07). The nucleation time increased statistically significant from 9.0 days to 20.4 (12 months) and 17.7 (24 months) days, respectively. Obviously, pharmacological effects of ursodeoxycholic acid exist which modify important processes in the gallstone's pathogenesis.