The distribution of parietal cells in the body mucosa, and of G cells in the antral mucosa, was quantitatively mapped in resected stomachs from 42 patients (12 with gastric ulcer, 11 with duodenal ucler, 14 with duodenal ulcer and uremia, and 5 with gastric cancer) who preoperatively had had their gastric acid secretion measured. In the material as a whole there was a significant positive correlation between the parietal-cell density and maximal acid output (MAO), and a significant negative correlation between the parietal-cell density and patient age. A significant positive correlation was found between the antral G-cell mass and basal acid output (BAO). When the individual patient categories were analyzed, the correlation between parietal-cell density and MAO were significant in the group with duodenal ulcer and uremia, and in the group with gastric cancer. Correlation between parietal-cell density and age was found only in the group with duodenal ulcer and uremia. There was no correlation between the parietal-cell density and various parameters of the antral G-cell population in the material as a whole or in any of the individual groups.