Thirty-five patients with fundic atrophic gastritis and achlorhydria were classified in two groups according to the presence or absence of fundic argyrophil, mostly enterochromaffinlike cell hyperplasia. Among the biologic and histologic parameters studied, the hyperplasic group differed only by a circulating hypergastrinemia and an antral G-cell hyperplasia. The histamine content, the histidine decarboxylase activity, and the mast cell number of fundic biopsies were determined in 10 controls, 16 of the preceding patients (11 with and 5 without fundic argyrophil-cell hyperplasia), and 5 patients with fundic atrophic gastritis and neither achlorhydria nor hyperplasia. Histamine content and histidine decarboxylase activity were increased only in the hyperplasic group despite an unchanged mast cell number. For all fundic biopsies the argyrophil-cell density was positively related to the histamine content. Finally, the argyrophil-cell hyperplasia occurring in fundic atrophic gastritis with achlorhydria is associated not with the gastritis intensity, as assessed by histologic and secretory criteria, but with a circulating hypergastrinemia and an increase of both fundic histamine content and histidine decarboxylase activity.