61 patients with radiolucent gallstones in gallbladders visible on oral cholocystography were treated with ursodeoxycholic acid (10 mg/kg/day) for 6 months. At the end of 6 months treatment, sonography of gallbladder and oral cholecystogram of a11 patients were interpreted by radiologist who had no prior knowledge of the treatment. Complete dissolution of gallstones occurred in 13 patients (21.3%) and partial dissolution occurred in 18 patients (29.5%). Ursodeoxycholic acid had no hepatotoxicity as assed by standard liver function tests. There were no discernible change in serurn cholesterol and triglyceride. There was not a significant difference between age and sex group in dissolution rate by treatment with ursodeoxycholic acid, however, there was a statistically significant different dissolution rate by size of gallstones, i.e. the smaller the stone size, the higher the dissolution rate. These results suggest that the ursodeoxycholic acid provides a safe and effective treatment for cholesterol gallstones and the rnost important factor influencing the effect of dissolution is the size of gallstones.