It is becoming increasingly apparent that peptic ulcer is not one disease, but a genetically, etiologically, and pathogenetically heterogeneous group of diseases that share a common endpoint, an ulcer crater in those parts of the gastrointestinal tract exposed to acid and pepsin. The evidence that both gastric and duodenal ulcer are each many different diseases is reviewed and a classification of the peptic diseases is proposed. The implication of this extensive heterogeneity is that each of the disorders leading to peptic ulcer may have a distinct etiology, pathophysiology, genetics, and natural history, and thus may be amenable to different modes of therapy and prevention. Only through the complete delineation of each of the disorders leading to peptic ulcer will specific diagnosis, prognosis, and treatment be possible for peptic ulcer patients and their families.