A new semielemental formula made up of whey protein hydrolysate, medium-chain triglyceride as 50% of fat, and glucose polymer was studied in 21 cystic fibrosis infants to determine whether its intestinal absorption was better than that of a standard milk-based formula. Each experiment lasted 10 days, during which the patient was fed for 5 days on one formula and immediately afterwards for 5 days on the alternative formula, without any pancreatic enzyme replacement. Fat and nitrogen absorption were assessed by 3-day balance studies, stool fat was assayed by a modified Van de Kamer method, and stool nitrogen by an automatic method. Ten infants with very severely impaired digestive function (coefficient of fat absorption on normal diet less than or equal to 75%) showed a highly significant improvement in fat and nitrogen absorption, leading to a significant gain in weight when fed on semielemental diet compared with standard diet. No significant improvement was observed in fat and nitrogen absorption or in weight gain in the 11 infants with less severe malabsorption (coefficient of fat absorption on normal diet greater than 75%). These results were obtained over a short period and have to be confirmed over a prolonged period of study with pancreatic enzyme supplementation. However, they suggest that such a semielemental diet should be effective for short-term treatment in selected CF infants who have difficulty in starting to thrive adequately.