PURPOSE: To evaluate the usefulness of the helical CT in the differentiation of periampullary malignanttumors. MATERIALS AND METHODS: Fifty-five periampullary carcinoma patients (pancreatic head carcinoma (n=18);distal CBD carcinoma (n=17) ; carcinoma of the ampulla of Vater(n=16) ; periampullary duodenal cancers,(n=4), alldiagnosed by histopathologic study] underwent helical CT with 5mm scan thickness and 5mm/sec table speed. Afterscanning, retrospective reconstruction was performed at 2mm intervals, followed by multiplanar reformation. Inboth retrospective reconstructed axial and multiplanar reformation images, the authors analyzed the detection rateand size of the mass, and associated findings including invasion of peripancreatic fat, dilatation of CBD and itsnarrowing pattern, dilatation of the pancreatic duct and its degree of dilatation, wall thickening of CBD,extension of dilated bile duct into the ampulla, and of protruding mass into the duodenal lumen, and lymph nodemetastasis all according to the origin sites of tumors. Differential points were thus determined. RESULTS: Thedetection rate of the masses was 96% (53/55). Their size was 1-5cm, with a mean size of 2.4 +/-0.5cm in carcinomaof of ampulla of Vater and 3.5 +/-1.0cm in pancreatic head carcinoma. Invasion of peripancreatic fat was mostcommonly observed in pancreatic head carcinoma (100%, 18/18) (P >0 . 0 5 ), dilatation of CBD was observed in allcases except one of periampullary duodenal cancer (98%, 54/55), and abrupt termination of dilated bile duct wasnoted in all cases except one of the pancreatic head carcinoma (98%, 53/54). Dilatation of pancreatic duct wascommonly observed in pancreatic head carcinoma (94%, 17/18) and carcinoma of the ampulla of Vater (75%, 12/16).Its degree of dilatation was mostly moderate in pancreatic head carcinoma (56%, 10/18) and mostly mild incarcinoma of the ampulla of Vater (63%, 10/16) ( P >0.05). Wall thickening of the distal CBD was most commonlyobserved in distal CBD carcinoma (76%, 13/17). Extension of dilated bile duct into the ampulla was commonlyobserved in the carcinoma of the ampulla of Vater (81%, 13/16) and periampullary duodenal cancer (75%, 3/4) (P>0.05). A mass protruding into the duodenal lumen was commonly observed in periampullary duodenal cancer (100%,4/4) and carcinoma of the ampulla of Vater (94%, 15/16) (P >0.05). Lymph node meatastasis was observed inpancreatic head carcinoma (17%, 3/18) and distal CBD carcinoma (6%, 1/17). CONCLUSION: Because of improvement inthe rate at which the mass is detected, and a clear demonstration of associated findings, helical CT is useful inthe differentiation of periampullary carcinomas.