Backgroud/Aims: Ultrasonography is the easiest, fastest procedure for the diagnosis of choledocholithiasis, but the diagnostic failure are mostly because of the intrapancreatic level of this condition and the absence of bile duct dillatation. Endoscopic ultrasonography is a promising procedure for the diagnosis of extrahepatic cholestasis due to noninvasiveness and accuracy in the digestive gas interposition. The aim of this study was to prospectively compare the diagnostic accuracy of endoscopie ultrasonography with abdominal ultrasonography and endoscopic retrograde cholangiography in 66 patients with suspected choledocholithiasis. Methods: All of the patients had abdominal ultrasonography, endoscopic ultrasonography ~and endoscopie retrograde cholangiography within 72 hours and final diagnosis was determined by endoseopic retrograde cholangiography with sphincterotomy. Results: (l) Choledocholithiasis was confirmed in 45 patients: Thirteen patients had nondilated common bile ducts and 20 patients had stones with diamerer < 1 cm. (2) Endoscopic ultrasonography was more sensitive(95.5%) than abdominal ultrasonography (44.4%: P<0.0001) and similar to endoscopic retrograde cholangiography (97.8%). Specificity was same as 100% in 3 diagnostic tools. (3) Endoscopic ultrasonography was superior to abdommal ultrasonography for the diagnosis of choledocholithiasis with nondilated common bile duct(100% vs 15.3%, P<0.0001) and with less than 1cm sized small stone(100% vs 10.0%, P<0.0001). Endoscopic ultrasonography results did not depend on stone diameter or common bile duct dilatation. Conclusions: Endoscopic ultrasonography could be an accurate and safe diagnostic tool for the diagnosis of choledocholithiasis and could be promising especially in the cases with difficult cannulation of bile duct and gal1 stone pancreatitis. (Korean J Gastrointest Endosc 17: 23-31, 1997)