The relationship between duodenitis and the outcome of duodenal ulcers was prospectively investigated by evaluating the frequency and extent of bulbar duodenitis before and after short-term medical treatment of the ulcer. Duodenitis appeared to be more frequent and more severe in the bulbar area close to the ulcer and was more widespread in the bulb of patients whose ulcer did not respond to treatment. After medical treatment, duodenitis improved only in the ulcer area and only in patients with complete ulcer healing. While confirming that, in some patients, duodenitis may well be an inflammatory reaction to the presence of the ulcer, data from the present study indicate that, in other patients, duodenitis, throughout the bulb, does not seem to be influenced by the outcome of the ulcer or the treatment. These findings suggest that there are subpopulations of duodenal ulcer patients who differ according to the pattern of bulbar duodenitis.