Each Disease affecting the bile ducts tends to produce characteristic pattern of billiary dilatation: recurrent pyogenic cholangitis causes dilatation and straightening of the larger(central) intrahepatic ducts ; clonorchiasis causes dilatation of the smaller (peripheral) intraahepatic ducts; and carcinoma along the extrahepatic ducts causes (proportional) dilatation and tortuosity of both larger and smaller intrahepatic ducts. To evaluate the specificity of the pattern and morphology of the dilated biliary tree on CT scancs (CT characterization) three independent radiologists who were unfamiliar with the cases were asked to classify 62 CT scans in patients with obstructive jaundice. The case population consisted of 14 cases with recurrent pyogenic cholangitis, 18 cases with clonorchiasis and 30 cases with carcinoma along the extrahepatic ducts, which were intermixed randomly. Classification was made only on the basis of CT characterization: those scans showing primary lesions i.t., stone, aggregate of flukes, or tumor mass were excluded or masked. All the scans of every case showing the extrahepatic bile duct were masked. Radiologists correctly classified 54 of the 62 cases (87%): ten of the 14 patients with recurrent pyogenic cholangitis(71%), 17 of the 18 patients with clonorchiasis(94%) and 27 of the 30 patients with carcinoma along the extrahepatic bile cucts(90%). We believe that CT characterization of bile duct dilatation is useful in the differential diagnosis of obstructive jaundice, especially when a primary pathologic lesion is not depicted in CT scans.