A clinical, endoscopic, and histological study of 206 cases of nodular hyperplasia of Brunner's glands was carried out. Firm nodules with a reddened surface due to hyperplastic Brunner's glands were limited mainly to the first part of the duodenum and affected almost exclusively male patients. Gastric acid secretion after pentagastrin stimulation was significantly increased compared to normal. In most cases, biopsies of the nodule center revealed spreading of Brunner's glands from within the lamina propria to the surface epithelium, whereas in biopsies performed between nodules, Brunner's glands were limited to the deeper part of the mucosa. Thirty-six nodules completely removed by diathermy were composed almost entirely of Brunner's glands. The frequent association with duodenal ulcer, chronic gastric erosions, and cobblestone pattern of the gastric body mucosa, as well as the significant hypersecretory state, suggest that hyperacidity plays a role in the pathogenesis of nodular hyperplasia of Brunner's glands.