Thirty-three insulin-treated men with diabetes were hospitalized on a metabolic ward and fed control diets (43% carbohydrate) for 6 to 11 days followed by high carbohydrate (70%), high fiber (HCF) diets for 12-35 days. Fasting blood glucose, cholesterol and triglyceride values were significantly lower on HCF diets than on control diets despite significantly (p less than 0.01) lower insulin doses on the HCF diets. HCF diets were accompanied by increased insulin sensitivity and by binding of insulin by monocytes. Patients who responded well to the diet in the hospital have maintained comparable glucose, cholesterol and triglyceride values as well as lower insulin doses for an average of 20 months on maintenance diets (60% carbohydrate). The high carbohydrate and low fat content of these HCF diets seem to play the predominant role in the improved glucose metabolism of these patients whereas the high plant fiber content may be responsible for the reduction in serum cholesterol and triglyceride values. These studies suggest that HCF diets may have an important place in the management of patients with the maturity-onset type of diabetes.