Effects of variations in dietary fat and carbohydrate content on various aspects of glucose, insulin, and lipoprotein metabolism were evaluated in 11 patients with hypertension, who also had non-insulin-dependent diabetes mellitus (NIDDM). All of these patients were being treated with sulfonylureas, thiazides, and beta-adrenergic receptor antagonists. The comparison diets contained either 40 or 60% of total calories as carbohydrate, with reciprocal changes in fat content from 40 to 20%. The diets were consumed in a random order for 15 days in a crossover experimental design. The ratio of polyunsaturated to saturated fat and total cholesterol intake were held constant in the two diets. Plasma glucose and insulin concentrations were significantly (P less than .001) elevated throughout the day when patients consumed the 60% carbohydrate diet. Fasting plasma total and very-low-density lipoprotein (VLDL) and triglyceride (TG) concentrations increased by 30% (P less than .001) after 15 days on the 60% carbohydrate diet. Total plasma cholesterol concentrations were similar on both diets, as were low-density lipoprotein (LDL) and high-density lipoprotein (HDL) cholesterol concentrations.