The first-degree relatives of duodenal ulcer patients and of control probands were evaluated clinically and by gastroduodenal endoscopy for prevalence of duodenal ulcer. The control probands were randomly selected from a control population. 199 relatives of 51 duodenal ulcer probands were interviewed, and 154 of these were endoscoped. 154 control relatives who had been endoscoped were matched with the DU relatives according to sex and age. Endoscopic evidence of present or past duodenal or pyloric ulcer was present in 20 (13.0%) of the DU relatives and in only 6 (3.9%) of the control relatives (p less than 0.01). The frequency of macroscopic duodenitis and gastric erosions was also significantly higher (p less than 0.05) in DU relatives than in controls. A history of epigastric pain was obtained in 54 (35.1%) of endoscoped DU relatives and in 24 (15.6%) of control relatives (p less than 0.01). This study has shown an increased prevalence of endoscopic evidence of duodenal ulcer in the first-degree relatives of duodenal ulcer patients. The finding that duodenitis is also more prevalent in DU relatives than in controls support the view that duodenitis is linked with duodenal ulcer.