Suppression of gastric acid forms the basis of treatment of duodenal and gastric ulcer, but the precise relationship between suppression of acidity and healing rates has not been defined. We examined the results of controlled trials and clinical pharmacological studies of 24-h intragastric acidity involving antisecretory agents. Data on 24-h and nocturnal hydrogen ion activity and nocturnal acid output were obtained, and the healing rates in duodenal ulcer were calculated. Duodenal ulcer healing rates after 4 weeks showed a significant correlation with suppression of 24-hour hydrogen ion activity (r = 0.63; P less than 0.05), and a highly significant correlation between healing and the suppression of nocturnal hydrogen ion activity (r = 0.93; P less than 0.0001). Nocturnal acid output was not significantly correlated. For gastric ulcer, no such association was seen for suppression of either 24-hour or nocturnal hydrogen ion activity. Duodenal ulcer is regarded as an acid-related disorder, but in gastric ulcer other factors may be more important in pathogenesis and treatment.