Serum gastrin responses to bombesin and to food were studied in patients with hypergastrinemia. Nine unoperated patients with hypergastrinemia of antral origin (seven achlorhydria by two idiopathic G-cell hyperfunction) had significantly higher serum gastrin responses to bombesin (P less than 0.001) and to food (P less than 0.001) than eight unoperated patients with Zollinger-Ellison syndrome. Three gastrectomized Zollinger-Ellison patients showed large serum gastrin responses to bombesin, and two of them had large postprandial increases in serum gastrin. Serum gastrin in a single patient with retained excluded antrum in the duodenal stump after partial gastrectomy with Billroth II anastomosis responded to bombesin but not to food. In nonoperated patients with hypergastrinemia of antral origin, the serum gastrin responses to bombesin and to food were significantly correlated (r = 0.786; P less than 0.02) while in unoperated Zollinger-Ellison patients no such correlation was found (r = 0.095; P greater than 0.10). It is concluded that determination of the serum gastrin response to bombesin does not provide more information than measurement of postprandial serum gastrin concentrations in the differential diagnosis of patients with basal hypergastrinemia.