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The accuracy of ultrasonography (US) for the diagnosis of cholelithiasis and for dilatation of the intra- and extra-hepatic biliary tree is well known. However, the value of US for the diagnosis of common bile duct stones remains poorly defined. We performed a prospective study in 100 patients who were referred for endoscopic retrograde cholangiopancreatography (ERCP); all the examinations were carried out by the same sonographist in the 24 h preceding the ERCP. Fifty patients had choledocholithiasis, 20 patients had obstruction of the bile ducts without lithiasis and the common bile duct (CBD) was free in 30 patients. The sensitivity of US for the diagnosis of choledocholithiasis was 40 p. 100, the specificity 90 p. 100. The positive and negative predictive values of the "CBD stone" sign was 80 p. 100 and 60 p. 100 respectively. In a total of 30 false negatives, the CBD could not be explored in 4 cases, dilatation of the CBD was missed in one case, and obstruction of the CBD by an other disease was diagnosed in 2; in all the other cases, US was able to appreciate the CBD size as well as the ERCP. In the 20 patients with an obstructed CBD but without choledocholithiasis, US diagnosed a stone in 5 cases. Age, serum bilirubin, existence of a previous cholecystectomy, technical difficulties, stone size were comparable in patients with true positive tests and in patients with false negative tests. However the diagnosis of choledocholithiasis was more frequently achieved in patients with dilated CBD over 10 mm (p less than 0.05) and in patients with multiple stones.(ABSTRACT TRUNCATED AT 250 WORDS)