To assess the effect of malnutrition on gastric acidity and gastric bacterial colonization, we studied 35 severely malnourished Bangladeshi children before (0 wk) and after (3 wk) they received nutritional rehabilitation for 3 wk. These results were compared with those obtained from a similarly examined group of 20 better-nourished Bangladeshi children. Gastric acid output, both basal and after betazole stimulation, was significantly lower in the malnourished group at 0 wk compared with the better-nourished children (p < 0.01): basal 0.22 vs. 0.52 mEq HCl/h and stimulated 0.90 vs. 2.5 mEq HCl/h. Both the concentration of acid and the rate at which gastric juice was secreted were decreased in the malnourished group but serum gastrin levels were not significantly different. After 3 wk, the malnourished children had improved from 61% (± 9.0%; SD) to 81% (± 8.1%) of expected weight-forheight and were not significantly different than the better-nourished group (86% ± 11%). Nevertheless, gastric acid concentration remained depressed in the 3-wk group, although the rate of gastric juice secretion equaled levels observed in the betternourished group. None of the better-nourished children had detectable gram-negative bacterial colonization of their gastric juice. In contrast, 26 of 32 (81%) malnourished children at 0 wk were colonized-even after betazole stimulation, 11 of 33 (33%) gastric juice samples yielded viable organisms-suggesting that the decrease in gastric acid output greatly reduced the gastric acid barrier. Interestingly, only 9 of 20 (45%) better-nourished children had gastric juice with basal pH values below 4.0, suggesting that the gastric acid barrier may be an intermittent defense factor in Bangladeshi children. © 1988.