In the United States about four million people have active peptic ulcers and about 350,000 new cases are diagnosed each year. Four times as many duodenal ulcers as gastric ulcers are diagnosed. Approximately 3000 deaths per year in the United States are due to duodenal ulcer and 3000 to gastric ulcer. There has been a marked decrease in reported hospitalization and mortality rates for peptic ulcer in the United States. Changes in criteria for selecting the underlying cause of death might account for some of the apparent decrease in ulcer mortality rates. Hospitalization rates for duodenal ulcers decreased nearly 50 per cent from 1970 to 1978, but hospitalization rates for gastric ulcers did not decrease. Although this decrease in hospitalization rates may reflect a decrease in duodenal ulcer disease incidence, it appears that changes in coding practices, hospitalization criteria, and diagnostic procedures have contributed to the reported declines in peptic ulcer hospitalization and mortality rates. There is no good evidence to support the popular belief that peptic ulcer is most common in the spring and autumn. The most consistent pattern appears to be low ulcer rates in the summer. There is strong evidence that cigarette smoking, regular use of aspirin, and prolonged use of steroids are associated with the development of peptic ulcer. There is some evidence that coffee and aspirin substitutes may affect ulcers, but most studies do not implicate alcohol, food, or psychological stress as causes of ulcer disease. Genetic factors play a role in both duodenal and gastric ulcer. The first-degree relatives of patients with duodenal ulcer have a two- to threefold increase in risk of getting duodenal ulcer and relatives of gastric ulcer patients have a similarly increased risk of getting a gastric ulcer. About half of the patients with duodenal ulcer have elevated plasma pepsinogen I. A small increase in risk of duodenal ulcer is found in persons with blood group O and in subjects who fail to secrete blood group antigens into the saliva. In most Western countries, morbidity from duodenal ulcer is more common than from gastric ulcer, even though deaths from gastric ulcer exceed or equal those from duodenal ulcer. In Japan, both morbidity and mortality are higher for gastric ulcer than for duodenal ulcer.