Augmented histamine tests were performed in 349 patients with duodenal ulcers verified by operation, of whom 268 were men between 16 and 84 years of age, and 81 were women between 12 and 78 years; all were citizens of Copenhagen. In the men the mean peak acid output was 44 mEq H+ per hour, ranging from 16.8 to 111.4 mEq. The mean acid output in women was 32.3 mEq H+ per hour, ranging from 13.7 to 74.4 mEq. In men with duodenal ulcer a statistically significant correlation between age and peak acid output is clearly demonstrable, the acid output decreasing by 5 mEq H+ per hour for each ten years’ increase of age. In women, the acid output falls about two mEq H+ per hour per decade, although the statistical significance of this finding is not certain.