Net external acid balance was studied in 12 malnourished infants with chronic diarrhea (some of whom had acquired monosaccharide intolerance). When the infants achieved an adequate energy intake from a formula that contained either glucose or glucose polymers, seven developed metabolic acidosis and five remained free of acidosis. During the study, the acidotic infants produced a significant excess of acid (3.7 +/- 2 vs. 0.5 +/- 2 mEq/kg/day, p less than 0.005). The amount they excreted in urine (2.9 +/- 2 mEq/kg/day), however, was similar to that excreted by nonacidotic infants (2.7 +/- 2 mEq/kg/day) and indicated renal inability to reduce the excess acid load. The net effect was hydrogen ion (H+) retention (+0.8 +/- 0.8 vs. -2.2 +/- 0.8 mEq/kg/day, p less than 0.001). Good correlation existed between the net acid balance and the acid-base measurement in the blood. We speculate that (a) the increased acid load was a consequence of colonic bacterial production of volatile fatty acids from carbohydrate malabsorbed from the small bowel and (b) the renal incapacity to excrete H+ probably was secondary to potassium and phosphate depletion.