It has been suggested that gastrin may be a causative factor in the proliferation of gastric fundic mucosal endocrine cells, as seen in the Zollinger-Ellison syndrome and in atrophic gastritis with hypergastrinemia of antral origin. In the present study, morphometrically determined densities of endocrine cells in fundic mucosal biopsy specimens were related to basal levels of serum gastrin in 10 normal controls and 60 patients with achlorhydric fundic atrophic gastritis, of which 45 had pernicious anemia (5 with fundic mucosal carcinoid) and 15 had atrophic gastritis without pernicious anemia. The densities of fundic mucosal endocrine cells were positively related to the levels of serum gastrin (atrophic gastritis, rs = 0.65; atrophic gastritis and normal controls, rs = 0.72). The highest levels of serum gastrin were found in patients with carcinoid tumors (mean, 1659.3 pmol/l), followed by those in patients with focal hyperplasias (cluster formation) of endocrine cells (mean, 503.2 pmol/l) and those in patients without focal hyperplasias (mean, 304.4 pmol/l) (p = 0.03 and p = 0.04, respectively).