Background: We assessed the diagnostic usefulness of endoscopic ultrasonography (EUS) for choledocholithiasis. Methods: A prospective series of 155 patients with suspected choledocholithiasis all underwent EUS, conventional ultrasonography, CT, and ERCP. In 142 patients with a clear cholangiogram on ERCP, we analyzed the capability of EUS to image the extrahepatic bile duct and to identify choledocholithiasis, compared with ultrasonography and CT. Results: No complications were encountered in performing EUS. In 51 patients, ERCP demonstrated bile duct stones, which were confirmed at endoscopic sphincterotomy or surgery. The extrahepatic bile duct was wholly displayed in 96% by EUS, in 60% by ultrasonography, and in 80% by CT. EUS (96%) was more sensitive than ultrasonography (63%) and CT (71%) for detecting choledocholithiasis (p < 0.001). Although ultrasonography and CT were poorly diagnostic for choledocholithiasis in patients with small stones or those with a nondilated common bile duct, EUS was able to accurately detect choledocholithiasis regardless of the size of stones or the diameter of the bile duct. The specificity of EUS (100%) was higher than those of ultrasonography (95%) and CT (97%). Conclusions: EUS, a safe imaging procedure, is more accurate than ultrasonography and CT and may be as accurate as ERCP for diagnosing choledocholithiasis. (Gastrointest Endosc 1997;45:143-6.)