The effects of high-carbohydrate, high plant fiber (HCF) diets on glucose and lipid metabolism of 20 lean men receiving insulin therapy for diabetes mellitus were evaluated on a metabolic ward. All men received control diets for an average of 7 days followed by HCF diets for an average of 16 days. Diets were designed to be weight-maintaining and there were no significant alterations in body weight. The daily dose of insulin was lower for each patient on the HCF diet than on the control diet. The average insulin dose was reduced from 26 +/- 3 units/day (mean +/- SEM) on the control diets to 11 +/- 3 (P less than 0.001) on the HCF diets. On the HCF diets, insulin therapy could be discontinued in nine patients receiving 15 to 20 units/day and in two patients receiving 32 units/day. Fasting and 3-hr postprandial plasma glucose values were lower in most patients on the HCF diets than on the control diets despite lower insulin doses. Serum cholesterol values dropped from 206 +/- 10 mg/dl on the control diets to 147 +/- 5 (P less than 0.001) on the HCF diet; average fasting serum triglyceride values were not significantly altered on the HCF diets. These studies suggest that HCF diets may be the dietary therapy of choice for certain patients with the maturity-onset type of diabetes.