There are two distinct problems in patients with Zollinger-Ellison Syndrome (ZES): peptic ulcer diathesis and malignant tumors. Antisecretory drugs have allowed us to control the ulcer symptoms and acid output in 45 patients with ZES. We report here the initial seven patients selected for surgical exploration with the goal of removing their gastrinomas. Prior to surgery, an extensive and rigorous protocol to localize the gastrinoma was carried out, including hypotonic duodenography, abdominal ultrasonography, selective arteriography, portal vein sampling for gastrin, and computerized tomography. With this protocol of radiographic localization, gastrinomas were found in two of the seven cases and the syndrome was "cured" in three of the seven patients. The results also demonstrate that preoperative localization is not a substitute for careful surgical exploration as tumors were found in two patients in whom localization failed.