The thickness of Brunner's glands was measured with an ocular micrometer in 75 cases of surgically resected duodenal ulcer and in 75 autopsy cases (control group). Endoscopic findings before operation were also studied. Maximum mean thickness of Brunner's glands in the control group was 1.54 +/- 0.38 mm (mean +/- SD), and no regional difference in thickness was noted. The thickness of Brunner's glands in the duodenal ulcer group was widely distributed, from 0.5 mm to 5.0 mm, and the average value was 3.0 +/- 1.0 mm, with a statistically significant difference from that of the control group. In most duodenal ulcer cases Brunner's glands were thickest within 1 cm from the center of an ulcer. Only six cases of duodenal ulcer (8.0%) showed a diffusely thin layer of Brunner's glands, less than 1.5 mm thick. These results show that the Brunner's glands become hyperplastic in duodenal ulcer patients, especially near the ulcer. In the healed ulcer Brunner's glands were thin at the center of an ulcer scar, and the average thickness was 0.42 +/- 0.26 mm. This histologic finding corresponded to the depressed scarred area observed endoscopically, suggesting a decreased mucosal resistance at this area.