Forty-five patients with achlorhydria due to severe atrophic corpus gastritis or gastric atrophy were studied by determination of serum gastrin, histological examination of multiple biopsy from the antrum, and quantitation of gastrin cells revealed by an indirect immunofluorescence technique. In a reference group of 12 persons with normal gastric secretion and without atrophic antral gastritis the mean number of gastrin cells per field of vision was 52 +/- 6.5 (S.E.M.). In a group of achlorhydric patients having normal antral mucosa (n = 24), the serum gastrin levels was 324 +/- 56 pmol/l and the number of gastrin cells was 79.6 +/- 7.5 cells/field of vision. The corresponding values for a group of achlorhydric patients with chronic superficial antral gastritis (n = 11) were 361 +/- 186 pmol/l and 88.0 +/- 14.4 cells/field of vision. In a group of achlorhydric patients with atrophic antral gastritis (n = 10) serum gastrin was 15.0 +/- 3.3 pmol/l, and the number of gastrin cells was 6.2 +/- 3.3 cells/field of vision. Compared to the subjects in the reference group, the number of gastrin cells was significantly higher in the groups of achlorhydric patients with normal or superficially inflamed antral mucosa and significantly lower in achlorhydric patients with atrophic antral gastritis. It is concluded that serum gastrin in general is a good indicator for the presence or absence of antral atrophic gastritis in achlorhydria.