TUDCA has been proposed in the treatment of hepatobiliary disease, but data on the enrichment of the biliary TUDCA pool and changes of biliary lipids after administration of the compound are scarce. We studied the composition of biliary lipids in a series of 33 patients with radiolucent stones, before and after treatment with TUDCA, 3.5 to 16.6 mg/kg/die for 4-6 weeks. Duodenal bile was collected with the Entero-Test after gallbladder contraction. Cholesterol saturation was 147% + 67 before treatment. TUDCA administration produced the following dose-dependent effects: a) a linear decrease of cholesterol saturation (r = 0.59; p); b) a non-linear increase of the percent of ursodeoxycholate in bile (r = 0.59; p); c) a non-linear increase of the fraction of ursodeoxycholate conjugated with taurine. At the dose of 11 mg/kg per day, cholesterol saturation was 80%, ursodeoxycholic acid represented about 45% of biliary bile acids, and about half of urso was conjugated with taurine. These results can be used as guidelines to assess the required daily dosage of TUDCA.