A prospective, double-blind, randomized, single-center study was conducted to compare ursodeoxycholic acid alone with the combination of ursodeoxycholic acid and chenodeoxycholic acid for dissolution therapy of gallstone fragments after shock wave lithotripsy. Patients with single radiolucent gallstones up to 30 mm in diameter or up to three stones of similar total volume received either 750 mg ursodeoxycholic acid alone (group A, n = 138) or the combination of 500 mg ursodeoxycholic acid and 500 mg chenodeoxycholic acid (group B, n = 144) in a single bedtime dose. The bile acids were administered from 2 wk before electrohydraulic lithotripsy until 3 mo beyond complete disappearance of all fragments. Patient's characteristics, stones (group A, 82% single stones, maximum diameter 19 +/- 5 mm, mean +/- S.D.; group B, 82% single stones, 18 +/- 5 mm), lithotripsy treatment and follow-up period were not different between the two groups. Between the two groups, no statistically significant difference was found in the time required for complete clearance of the fragments (group A, median time = 15 mo; group B, median time = 13 mo; p = 0.7). At 12 mo after lithotripsy, the probability of complete clearance was 46% +/- 5% in group A and 49% +/- 5% in group B. Diarrhea occurred significantly more often in group B than in group A (p less than 0.001) and was the main reason for withdrawal of randomized medication. Severe adverse effects of the bile acids were not observed. It is concluded that monotherapy with ursodeoxycholic acid is as efficient as the combination of ursodeoxycholic acid and chenodeoxycholic acid for fragment dissolution after lithotripsy.(ABSTRACT TRUNCATED AT 250 WORDS)