Biopsies were obtained from non-ulcerated sites of the duodenum from 100 dyspeptic patients. Helicobacter (Campylobacter) pylori was cultivated from 19 of these biopsies. Active chronic duodenitis (ACD) was found in 17 biopsies and more than 5% gastric metaplasia in 20 biopsies. H. pylori as well as ACD occurred with a significantly increased frequency when more than 5% gastric metaplasia was found in the duodenal biopsies. H. pylori on metaplastic tissue without ACD was, however, seen in two cases. H. pylori was cultivated from 9% and ACD was found in 5% of the biopsies with less than 5% gastric metaplasia. Gastric metaplasia in the duodenum was found significantly more frequently in patients with endoscopic duodenitis or duodenal ulceration than in patients with normal endoscopy. No association between gastric metaplasia in the duodenum and gastric pH or serum antibodies against H. pylori was seen. This study indicates that there is an established, but not exclusive, connection between gastric metaplasia and the colonization of the duodenum by H. pylori, the most important role being played by the antral gastric mucosa rather than the duodenum.