Summary The incidence of peptic ulceration in male patients with chronic bronchitis and emphysema has been compared with that in men of similar age suffering from other diseases. Nineteen per cent of 26 patients with pulmonary heart failure had clinical evidence of chronic peptic ulceration, compared with 13 per cent of 159 emphysematous patients with no history of heart failure and 12 per cent of 81 men with other forms of heart disease. At postmortem examination 7 per cent of 43 patients with cor pulmonale had chronic peptic ulceration, compared with 10 per cent of 100 patients with other forms of heart disease and 12 per cent of 73 patients without heart disease. Nineteen per cent of the patients with cor pulmonale, 12 per cent of the patients with other forms of heart disease, and 5 per cent of the patients without heart disease had acute peptic ulcers. There was no significant difference between the CO 2 tension and oxygen saturation of the arterial blood of 30 emphysematous patients with peptic ulcers and 89 similar patients who had no evidence of peptic ulceration. There was also no difference between the rate of gastric secretion of acid by 12 men in whom the partial pressure of CO 2 in the arterial blood was less than 46 mm. of Hg and 16 men in whom the CO 2 tension was higher than this. Thus, no evidence has been found to support the hypothesis that chronic respiratory acidosis causes either an increased secretion of acid by the stomach or an increased incidence of chronic peptic ulceration.