This study was designed to determine the effects of varying the proportions of carbohydrate, fiber, and fat on metabolic control in Type II diabetes. Ten men, aged 50 to 69 years, with Type II diabetes participated. Four isocaloric diets were consumed for 2 weeks each, with a break of 6 to 14 weeks between diets to ensure no carryover effects. Two of the diets were high in carbohydrate (63% to 65% energy) and low in fat (10% to 12% energy) but differed in their fiber contents (20 vs. 45 gm/day). The other two diets were low in carbohydrate (23% to 27% energy) with either a low or a high fat content (15% vs. 55% energy) and a high or normal protein content (62% vs. 18% energy). The composition of the subjects' usual diets in the week before each of the experimental diets did not vary significantly: carbohydrate 47% to 50% energy, protein 22% to 25% energy, fat 27% to 31% energy, and fiber 24 to 25 gm/day. A 75-gm oral glucose tolerance test and a 12-hour metabolic profile in response to 3 meals typical of the particular diet were conducted before and at the conclusion of each 2-week dietary period. The most significant improvements in metabolic control (as assessed by the effects of the diets on fasting glucose and on lipids, and on the glucose and insulin responses to oral glucose and the mixed meals) were obtained with the high-fiber, high-carbohydrate, low-fat diet and with the low-carbohydrate, high-protein, low-fat diet. Metabolic control was not significantly affected by the low-fiber, high-carbohydrate, low-fat diet, but it deteriorated significantly on the low-carbohydrate, high-fat diet. The results of this study confirmed the importance of high fiber and low fat in improving metabolic control in Type II diabetes. In conclusion, if high-carbohydrate, low-fat diets are to be recommended to patients with diabetes, it is essential that the type of carbohydrate recommended be unrefined and high in fiber.