Eighteen cases of severe hyperplasia of fundic argyrophil cells observed during routine histologic examination of endoscopic gastric biopsy specimens from unselected patients with upper gastro-intestinal symptomatology were investigated. All patients, except one, were female with a mean age of 57 years. Atrophic gastritis of fundic mucosa with severe hypo- or achlorhydria was present in all cases. Hypergastrinaemia (of antral origin) was found in 15 subjects in which circulating gastrin levels were determined. Pernicious anaemia was seen in 1 patient. At light microscopy, the hyperplastic fundic cells were stained by the Grimelius and the Sevier-Munger silver methods and, in approximately 30% of cases, by lead-haematoxylin. In addition, these cells reacted with anti chromogranin antibodies. In 8 of 9 patients studied by electron-microscopy, enterochromaffin-like (ECL) cells were found to be the more frequent cell type. D1 cells prevailed in 1 case and were rare in the others. The frequency of P cells was intermediate between that of ECL cells and that of D1 cells. In conclusion, our observations indicate that: argyrophil cell hyperplasia of atrophic fundic mucosa is prevalently found in women with hypergastrinaemia, and the hyperplastic process involves mostly the ECL type of gastric endocrine cells. It is noteworthy that similar associations have been shown to be present in patients affected by fundic carcinoid tumours and atrophic gastritis.