Epidemiological evidence indicates that duodenal ulcers are caused by environmental ulcerogens, although the nature of these ulcerogens has not yet been defined. It is not known whether individuals who develop an ulcer do so because they are uniquely susceptible to the ulcerogens or whether the ulcerogenesis is a chance event. It seems that the main abnormality underlying duodenal ulceration relates to failure of normal healing, as reflected by chronicity and tendency to recurrence. It is not yet known whether there is failure of the repair processes, or interference with the processes involved in normal mucosal repair.