The present prospective study was done to compare the size of the extrahepatic bile duct as measured by ultrasound with those measured by different radiographic techniques. The sonographic diameters were significantly correlated to the diameters measured by intravenous cholangiography (r = 0.92), to the diameters measured by endoscopic retrograde cholangiography (r = 0.81), and to the diameters measured by percutaneous transhepatic cholangiography (r = 0.85). The mean radiographic diameters were significantly greater for intravenous cholangiography, endoscopic retrograde cholangiography, and percutaneous transhepatic cholangiography than the mean sonographic diameters. Analyzing the regression lines between the radiographic and sonographic diameters and studying the sonographic diameters during intravenous cholangiography, the discrepancy between sonographic and radiographic measurements appears to be due to several factors: (a) radiographic magnification, (b) choleretic effects in intravenous cholangiography, (c) sonographic minification, (d) dilatation of the extrahepatic bile duct due to direct injection of a contrast agent in endoscopic retrograde cholangiography and percutaneous transhepatic cholangiography, and (e) premedication in endoscopic retrograde cholangiography and percutaneous transhepatic cholangiography. Because the normal size of the extrahepatic bile duct is not usually assessed in healthy subjects by intravenous cholangiography, endoscopic retrograde cholangiography, and percutaneous transhepatic cholangiography, the mean diameter and the upper normal limit of the extrahepatic bile duct were calculated for intravenous cholangiography, endoscopic retrograde cholangiography, and percutaneous transhepatic cholangiography using previous sonographic data from healthy subjects and the present regression lines. The calculated upper limit for normal was 7-8 mm in intravenous cholangiography and 10-11 mm in endoscopic retrograde cholangiography and in percutaneous transhepatic cholangiography.