SUMMARY Peak acid outputs in normal subjects and patients with duodenal and gastric ulcer were significantly (and similarly) correlated with body weight and calculated lean body mass.Expression of measurements of peak acid output as a function of body weight or lean body mass lessened but did not abolish the significance of the situation that men with no dyspepsia or gastric ulcer secreted more acid than women; the significant sex difference in duodenal ulcer was, however, abolished.Data for peak acid output have been tabulated, not only as m-equiv/hr but also as g-equiv/kg body weight/hr and g-equiv/kg lean body mass/hr. On the basis of the present data no change is suggested in the current expression of peak acid output as m-equiv/hr since expression of acid output as a function of body weight or lean body mass improves neither the variability nor the diagnostic discrimination of measurement of peak acid output.There have been many attempts to relate gastric secretion to different measurements of body build. The simplest hypothesis is that maximum acid output (Kay, 1953) as a function of the parietal cell mass of the stomach (Card and Marks, 1960) is a function of body weight, and significant correlations have been found between maximum acid output and body weight in animals, such as the rat (Shay, Sun, and Gruenstein, 1954) and the dog (Baron, unpublished data), as well as the human child (Ghai, Singh, Walia, and Gadekar, 1965) and infant (Rodbro, Krasilnikoff, and Christiansen, 1967), with no differences between the sexes.