The authors retrospectively reviewed computed tomographic (CT) scans, angiograms, and surgical-pathologic records of 226 patients without and 64 patients with pathologically proved pancreaticobiliary carcinoma to evaluate frequency of depiction, normal range of size, and causes of dilatation of the posterior superior pancreaticoduodenal vein (PSPDV) at CT. Among the patients with pancreaticobiliary carcinoma, CT demonstrated enlarged PSPDVs (diameter, greater than or equal to 8 mm) in seven patients. CT and angiography showed that the portal-superior mesenteric vein (P-SMV) was occluded or stenotic at its confluence in three patients and was normal in four patients. Among the latter four patients, localized tumor invasion was found surgically-pathologically along the right lateral wall of the P-SMV in three. A dilated PSPDV with obliteration of the P-SMV at CT may confirm tumor extension to the P-SMV. In patients with a normal P-SMV at CT, a dilated PSPDV indicates that the tumor has extended beyond the pancreatic parenchyman and occluded other pancreaticoduodenal veins or extended to the wall of the P-SMV.