Eight patients with insulin-requiring diabetes were given, in random order, a diet containing 3 g and one containing 20 g of crude fiber. Each diet was maintained for 10 days and was the same in calories, carbohydrate, fat, and protein. Insulin dose was kept constant. Mean plasma glucose on the low-fiber diet was 169.4 +/- 11.7 mg/dl, significantly higher (P less than 0.001) than the value of 120.8 +/- 10.1 on the high-fiber diet. Hypoglycemic reactions were more common on the high-fiber diet. Weight remained essentially constant on both diets. Mean serum glucagon level on the high-fiber diet was significantly lower (P less than 0.001) than on the low-fiber diet, while serum free insulin levels were unchanged. These data indicate that substantial changes in fiber content of the diabetic diet may lead to marked changes in diabetic control and that increasing dietary fiber may be a useful means of lowering plasma glucose in some diabetic patients.