Determinations of collagen in specimens of duodenal mucosa taken from 15 patients operated on because of duodenal ulcer with pyloric stenosis were performed. In the ulcer and surrounding tissue, total collagen values were significantly increased when compared with the results in unchanged wall of the duodenum. Collagen polymorphism study showed considerably higher percentage of type I and decreased type III in the ulceration than in control duodenal mucosa. In conclusion, extracellular components of connective tissue may play a role in the formation and course of duodenal ulcer disease.