Difficulty in excluding duodenal ulceration by standard radiology has hampered investigation of duodenitis; endoscopy and biopsy add a new dimension. We have studied 51 patients with dyspeptic symptoms or bleeding in whom endoscopy revealed congested and irregular areas of duodenal bulb mucosa in the absence of ulcer or scar. Biopsy revealed unequivocal active inflammation-epithelial cell atypia and neutrophil infiltration-in 22 of these patients, but in none of 29 "control" patients with an endoscopically normal bulb. There were no reliable radiological markers of active duodenitis. These patients, and a group of 25 with known duodenal ulcers, shared similar clinical characteristics and a high incidence of gastric metaplasia in the bulb. In the short follow-up period most patients with active duodenitis remained symptomatic; two developed ulcers, and two others have undergone vagotomy and pyloroplasty, with poor short-term results. The clinical significance of visual and histological mucosal changes, and their relationship with ulcer disease, merit long-term study.