A 37-year-old man with a primary mesenteric gastrinoma is reported. A parathyroid adenoma had been removed 13 years ago. Six years earlier the patient underwent an emergency distal gastrectomy because of massive bleeding from a duodenal ulcer. Two months later stomal ulcers developed associated with hypergastrinemia. No gastrinoma was detected and total gastrectomy was performed. The fasting plasma level of gastrin was stable in the range from 580 to 920 pg/ml for the following 5 years. However, the level was found to abruptly increase to 4125 pg/ml. The level increased progressively to 11383 pg/ml within 1 year. A gastrinoma was identified in the jejunal mesenterium, and it was completely removed. After surgery the plasma level of gastrin decreased below the limit of the assay, and the paradoxical response to secretin was no longer observed.