Ten young adult cystic fibrosis (CF) patients over 16 years of age (average 21.4 years) began nighttime enteral feedings as a method of nutritional rehabilitation to regain and maintain body weight. Patients received nighttime feedings of 1,000 kcal/M2 of a low- (Pulmocare), medium- (Ensure Plus), or high-carbohydrate (Vivonex) formula for at least 2 nights each with pancreatic enzyme therapy. Five of ten young adult CF patients developed nocturnal hyperglycemia (serum glucose greater than 300 mg/dl) and glucosuria (1-3% glucose) with varying degrees of polyuria during enteral feedings. No patient developed ketonuria despite serum glucoses at times greater than 600 mg %. There was no difference between the hyperglycemic and normoglycemic groups in median age, percent of ideal body weight, NIH score, Brasfield scores, pulmonary function tests, or family history of diabetes. All normoglycemic and four of five hyperglycemic patients had normal fasting blood sugars. The percent hemoglobin A1c was greater in the glucose intolerant group than the normoglycemic patients (11.2 +/- 0.8% vs. 6.8 +/- 1.1%, mean +/- SE, p less than 0.005). Twelve to 15 units of NPH insulin prior to initiation of feedings provided adequate therapy in most hyperglycemic patients. There was no apparent difference in the elevation of early morning serum glucoses with the low- medium- and high-carbohydrate formulas. We concluded that hyperglycemia requiring insulin therapy was common in young adult CF patients using nighttime enteral feedings. A hemoglobin A1c appeared to be a useful screening test before initiating such therapy.