The Zollinger‐Ellison syndrome can be best diagnosed by the measurement of elevated concentrations of peripheral venous gastrin. Using the gastrin radioimmunoassay, ulcerogenic tumors were diagnosed preoperatively in 4 patients. In three of these patients, no tumor could be found at laparotomy. In each, a tiny lesion was found in the submucosa of the duodenum, one at subsequent laparotomy, one in the duodenal stump resected for suspicion of retained antrum, and the third at autopsy. In the fourth patient, the duodenum was opened and a tiny duodenal islet cell adenoma resected. In the second and fourth patients, resection of the adenomas appears to have cured the patients without total gastrectomy; they are asymptomatic and have markedly lowered gastrin levels postoperatively. A plan for the management of duodenal islet cell tumors is presented which suggests that total gastrectomy might possibly be avoided if the surgeon is confident that the tumor is confined to the duodenum; this should be verified by following the course of the concentrations of peripheral venous gastrin.