Heterogeneity is the most important consideration in the pathophysiology of peptic ulcer disease. Acute ulcers and erosions present clinically with gastrointestinal bleeding or perforation. If they heal there is no predictable recurrence. Factors concerned with mucosal defense are relatively more important than aggressive factors such as acid and pepsin. Local ischemia is the earliest recognizable gross lesion. The gastric mucosa is at least as vulnerable as the duodenal mucosa and probably more so. Most drug-induced ulcers occur in the stomach. Chronic or recurrent true peptic ulcers (penetrating the muscularis mucosae) usually present with abdominal pain. Many duodenal ulcer patients report that the pain occurs when the stomach is empty or is relieved by food, and follows a pattern of relatively long periods of freedom from symptoms between recurrences. Approximately 50% of patients experience a recurrence within a year if anti-ulcer medication is stopped. In most western countries recurrent duodenal ulcer is more common than gastric ulcer. Peptic ulcer disease is also more common in men. Recent evidence indicates genetic and familial factors in duodenal ulcer and increased acid-pepsin secretion in response to a variety of stimuli. However, it is also becoming clear that of all the abnormal functions noted, few are present in all subjects and many are clustered in subgroups. In chronic gastric ulcer of the corpus, defective defense mechanisms, such as duodenogastric reflux and atrophic gastritis, seem to be more important than aggressive factors. Nevertheless, antisecretory medications accelerate the healing of such ulcers. It remains to be seen whether prostaglandins, mucus secretion, or gastric mucosal blood flow are impaired in chronic ulcer disease.