A 53-year-old man with duodenal ulcer continued to show hyperacidity following vagotomy and pyloroplasty. Findings from serum gastrin assays were six times normal. Partial pancreatectomy and total gastrectomy relieved symptoms and lowered serum gastrin levels to about twice normal. The resected pancreas, grossly normal, showed on microscopic examination an abnormally increased amount of islet tissue. The stomach showed an increased number of parietal cells. The patient is a rare example of the Zollinger-Ellison syndrome due to hyperplasia of the pancreatic islets.