PURPOSE: To assess the efficacy of ancillary CT findings other than the obstructive lesion per se for the differential diagnosis of extrahepatic biliary obstruction. MATERIALS AND METHODS:CT findings of 49 patients with extrahepatic bile duct obstruction(22 benign and 27 malignant lesions) were assessed with emphasis on the patterns of ductal dilatation, contrast enhancement, and diffuse thickening of the extrahepatic ductal wall. Degree of central and peripheral intrahepratic ductal dilatation was graded by comparing with the adjacent portal radicles and hepatic parenchymal thickeness. RESULTS: Diffuse circumferential thickening and contrast enhancement of the extrahepatic ductal wall were more frequent in benign cases, but only thickening was statistically significant(p < 0.01). Peripheral intrahepatic ducts were more severely dilated by malignant causes(p < 0.01). With the same degrees of extrahepatic and central ductal dilatations, peripheral intrahepatic ducts were more severely dilated in malignant than in benign cases. CONCLUSION: These results may help to interpret the CT findings of extrahepatic biliary obstruction, particularly when the cause of biliary obstruction is uncertain.