Thirty-three patients suspected of having a functioning tumor of the pancreas were studied with computed tomography (CT); 25 had angiographic studies also. Thirty-one cases were confirmed surgically; the other two were lost at follow-up and were excluded from the series. Of the 31 patients, 21 had insulinomas, five had Zollinger-Ellison syndrome, and five had assorted apudomas. CT was positive in 71% of the cases and negative in 29%; 22.6% of the studies were false negative and 6.4% were true negative. There were no false positives. Of the 21 patients with insulinomas, only one had false-negative CT and angiographic studies, caused by diffuse microadenomatosis. In one patient, two separate lesions of the pancreas were seen and identified correctly on both CT and angiography. All identified lesions smaller than 2 cm were seen only because of a hyperdensity on the enhanced CT scan. Lesions larger than 2 cm were identified either because their size distorted the pancreatic contour or because of their vascularity. The importance of proper CT technique using fast scanners and repeated bolus injections of contrast medium is stressed.