High carbohydrate diets rich in dietary fiber were fed to 13 hyperglycemic diabetic men; five men required 15 to 28 units of insulin per day, five men required sulfonylureas, and three men required 40 to 55 units of insulin. All 13 men were fed weight maintaining American Diabetic Association diets containing 43% of calories as carbohydrate for 1 week and then were fed 75% carbohydrate diets with 15 g of crude dietary fiber for approximately 2 weeks. After 2 weeks on the 75% carbohydrate diet, sulfonylureas were discontinued in all five men, insulin was discontinued in four men and decreased from 28 to 15 units in 1 man from the group requiring less than 30 units per day. Fasting plasma glucose values were significantly lower (P less than 0.001) in all 10 men. However, insulin requirements and fasting plasma glucose values were not changed in the three men requiring 40 to 55 units of insulin. Fasting serum cholesterol values were significantly (P less than 0.001) lower and mean fasting serum triglyceride values were 15% lower on the high carbohydrate diet than on the American Diabetic Association diet in these 13 men. Thus, a high carbohydrate diet with generous amounts of dietary fiber may be the treatment of choice of diabetic patients requiring sulfonylureas or less than 30 units of insulin per day.