To determine how the carbohydrate (CHO) content of "semielemental" formulas affects tolerance and macronutrient absorption, we enrolled 12 infants with severe diarrhea in two successive metabolic balance studies. The infants received, in random order, one of two isocaloric formulas that differed mainly in their CHO and fat concentrations. No significant differences were found between the two feeding periods for peak breath hydrogen levels and fecal osmolality. The low-CHO formula was tolerated better than the high-CHO formula, as indicated by a significantly lower stool output (mean +/- SD 387 +/- 230 vs 764 +/- 443 gm, respectively; p less than 0.05), and higher fecal pH (5.9 +/- 0.7 vs 4.9 +/- 0.5; p less than 0.05). Macronutrient absorption was greater during the ingestion of the low-CHO formula, as indicated by a significantly higher coefficient of fat absorption (p less than 0.005) and lower total fecal energy (mean +/- SD for high- vs low-CHO formula, 372 +/- 205 vs 207 +/- 102 kcal; p less than 0.05), which resulted from a lower CHO excretion (p less than 0.05). A correlation coefficient between the energy derived from CHO in feces and the total stool output was significant for both the high-CHO formula (r = 0.83; p less than 0.001) and the low-CHO formula (r = 0.7; p = 0.01). The CHO concentration of the special infant formula that we tested had an overriding effect on stool output and on fat and energy absorption.