The numbers of immunoreactive gastrin and somatostatin cells in gastric and duodenal mucosal biopsy specimens from dyspeptic patients with duodenal ulcers and dyspeptic controls without ulcers were calculated using a morphometric method. The levels of gastrin and somatostatin in the tissue were also measured by the radioimmunoassay. The results showed no significant difference in the number of G cells and the level of gastrin in the tissue between the ulcer and non-ulcer groups. However, the number of D cells and the level of somatostatin in the tissue in ulcer patients were remarkably reduced in comparison with those in non-ulcer patients (P less than 0.01 and P less than 0.05, respectively). The G:D cells and gastrin:somatostatin ratios in ulcer patients were much higher than those in the non-ulcer control group. It is considered that the reduction of D cells and the relative lack of somatostatin in duodenal ulcer patients might have a role in the mechanism of the duodenal ulceration.