The effects of a high-carbohydrate, high-fiber (HCHF) diet on glucose control was evaluated in 12 children with type I diabetes mellitus. The children had had diabetes for an average of 5.25 yr; their mean glycosylated hemoglobin was 12.4% (normal 5-9%), and C-peptide was virtually undetectable in all but one. They were followed on a regular diabetic diet for 10 days at home and in the hospital and then were studied on a HCHF diet for 14 days. The HCHF diet contained 60% carbohydrate and 30 g of fiber per 1000 cal provided through grains, fruits, vegetables, and high-fiber crackers. Capillary blood glucose levels were monitored at home before meals and at bedtime, and venous plasma glucose levels were measured in the hospital before and after each meal and during the night. Plasma glucose was measured serially after test meals with each diet. There was no significant difference in blood glucose levels preprandially, postprandially, and while fasting on the two diets. The 24-h glucose profiles and posttest meal profiles obtained during both diets were remarkably similar. We conclude that a diet high in fiber and carbohydrate has limited application in children with type I diabetes mellitus who have no residual beta-cell function.