With better methods of diagnosis, patients will be identified earlier in the course of their disease and will often have atypical and borderline manifestations of the syndrome. Serum gastrin measurements with calcium and especially with secretin challenge will be the most important method of diagnosis. Any patient with acid hypersecretion who has a high serum gastrin level that does higher on secretin infusion should be considered to have the Zollinger-Ellison syndrome. A firm diagnosis of the Zollinger-Ellison syndrome should be made, if at all possible, prior to operation. At operation, a thorough search of the pancreas, duodenum, stomach, greater and lesser omentum and liver should be made for primary and secondary gastrinomas. If the preoperative data firmly establish the diagnosis of the Zollinger-Ellison syndrome, a total gastrectomy should be carried out even if no primary tumor is found. Similarly, a total gastrectomy should be done even if there are massive hepatic metastases. If total gastrectomy is not performed, the patient is apt to die of complications of acid hypersecretion. The only possible exceptions to the rule of always performing a total gastrectomy are in asymptomatic patients with easily excisable tumors or patients with tumors of the duodenum that are easily excisable, providing that in both instances after the excision of the tumor the output of gastric acid as measured at operation is immediately halted. All possible metastatic tumor tissue should be removed. The more tumor tissue removed, the longer the patient will survive. Metastases should be treated aggressively. They do not disappear after total gastrectomy in our experience, and they may kill patients. Patients should be followed after operation with serial measurements of serum gastrin concentrations and by hepatic scintillation scans and hepatic angiography. If hepatic metastases develop, intrahepatic artery infusions of 5-fluorouracil may slow tumor growth.