Whereas 67 patients with duodenal ulcer had fasting and 30-minute postprandial mean serum gastrin levels not substantially different from 32 normal subjects, they had substantially higher fasting and histamine-stimulated gastric acid secretion. The increased acid secretion found in patients with duodenal ulcer is not caused by increased serum gastrin levels. Ten patients with recurrent ulcer, after incomplete vagotomy and gastric resection, had high gastric acid secretion and normal serum gastrin levels. Three patients with recurrent ulcer following complete vagotomy and gastric resection, but with retained antrum, had both high gastric acid secretion and high fasting and postprandial serum gastrin levels. Three patients with Zollinger-Ellison tumors had even higher basal acid outputs and serum gastrin levels. The combination of basic gastric acid secretory studies and serum gastrin determinations may identify three causes of recurrent ulcer: incomplete vagotomy, retained antrum, and Zollinger-Ellison tumor.