Basal and pentagastrin stimulated acid output was measured in 80 normal and 179 duodenal ulcer subjects of Chinese, Indian and Malay origin. Basal and maximally stimulated acid output was significantly higher in duodenal ulcer patients compared with normal subjects. There was however considerable overlap and less than one in four duodenal ulcer patients were hypersecretors. The acid output (and hence the parietal cell mass) was lower than in Caucasian subjects and this was possibly related to weight differences. The acid output did not differ significantly in the Chinese, Indian and Malay subjects, suggesting that parietal cell mass in the three racial groups is closely similar. The difference in frequency of duodenal ulcer disease in the three racial groups is thus not related to gastric secretory capacity.