Although acetylsalicylic acid (aspirin) can cause acute damage to human gastric mucosa, its relationship to chronic gastric ulcer has been less documented. A study of the aspirin intake of 61 patients with gastric ulcer revealed that 32 (52%) took 15 or more aspirins each week, compared to 6 (10%) age- and sex-matched controls who took such amounts (P less than 0.001). This difference remained significant when patients who took aspirin for relief of alimentary symptoms or when patients whose symptoms preceded aspirin use were excluded. The regular use of aspirin was especially associated with ulcers in the prepyloric region of the stomach. Fewer patients who had combined gastric and duodenal ulcers were regular users of aspirin when compared with patients who had gastric ulcer alone. The findings suggest that aspirin may be a common cause of gastric ulcer in man.