PURPOSE: To establish the criteria for differential diagnosis between malignant tumor and benign prominenceof papilla of Vater, as seen on CT. METHOD AND MATERIALS: Sixteen consecutive patients with prominent papilla ofVater, as seen on CT during a ten-month period were includedin this study. Final diagnosis was papilla of Vatercancer (n=5), chronic inflammation (n =3), benign tumor (n=3), or and normal (n=5), and this was confirmed bysurgery in 11 cases, and endoscopy in five. Papilla size and attenuation, the presence of accompanied dilatationof the bile or pancreatic duct, and lymph node enlargement were analyzed by two experienced radiologists, whoreached a conensus. A past history of stone disease, laboratory findings such as serum bilirubin, serum alkalinephosphatase, or endoscopic findings of duodenal diverticulum were additionally analyzed. RESULT: Papilla size wasthe only significantly different CT finding between malignant and benign lesions, and serum alkaline phosphataselevels were also significantly different between the two groups. The smallest malignant tumor was 18 mm and thelargest benign lesion was 15 mm. The presence of bile or pancreatic duct dilatation, serum bilirubin level,attenuation of the mass, a history of stone disease, and lymph node enlargement were not significantly differentbetween the two groups. CONCLUSION: In patients with prominent papilla of Vater, as seen on CT, a mass largerthan 18 mm is the only reliable radiologic finding to indicate malignant tumor of papilla of Vater. Serum alkalinephosphatase levels can, in addition, be helpful for the differential diagnosis of benign and malignant lesions.