In the course of a prospective study authors examined the role of the hyperplasia of Brunner's glands in the mucosal protection of proximal part of duodenum in patients with peptic ulcer, chronic pancreatitis and chronic renal insufficiency. Their method for this study was the histological examination of the endoscopic biopsy specimens. The hyperplasia of Brunner's glands occurs with significant frequency in all the three examined groups of patients, while we found it less frequently in the controls. Hyperplasia of Brunner's glands is one of the protective mechanisms of the organism, which serves the protection of the duodenal mucosa between the pyloric ring and the papilla of Vater, against the damaging effect of the hydrochloric acid.