Fundic argyrophil carcinoid tumors developed in the course of a 5-year continuous treatment with high dosages of H2-antagonists in a well-documented case of Zollinger-Ellison syndrome with primary hyperparathyroidism, high basal acid output, and serum gastrin. Approximately 100 small polyps were disseminated throughout the gastric fundus exclusively, leading to total gastrectomy. Metastatic carcinoid in a lymph node and pancreatic gastrinomas also were found at surgery. Gastric endocrine cell proliferation varied from simple argyrophil cell hyperplasia to carcinoid tumors eroding the surface and infiltrating the submucosa. Ultrastructural studies showed that the tumoral proliferation was heterogeneous, and included tumors composed of enterochromaffin (EC) and typical enterochromaffin-like (EC-L) cells, and tumors in which a majority of cells exhibited dense round granules resembling those of A-like or D1/P endocrine cell types. The risk of developing gastric fundic carcinoid tumors in ZES patients submitted to long-term antisecretory treatment should be given increased attention.