Preoperative angiography was performed on 10 patients who had insulin-producing islet cell adenomas. Eight of the 10 islet cell tumors were localized. Seven of the eight adenomas were demonstrated only on subselective injections into the small intrapancreatic arteries, and one was demonstrated by a selective injection into a large artery. Failure to localize two islet cell adenomas was probably caused by the inability to subselect the small intrapancreatic arteries supplying the tumor. One tumor was seen only with subtraction techniques, which were used in all cases. Localization was not related to vascularity, size of tumor, or location within the pancreas. Vasoconstrictive pharmacoangiography was not helpful; magnification was helpful but not essential. The most important factor in localizing islet cell tumors is demonstrating the complete pancreatic blood supply with subselective injections into the small intrapancreatic arteries supplying the tumor (inferior and superior pancreaticoduodenal and dorsal arteries).