Syndrome X refers to a cluster of abnormalities, associated with resistance to insulin-mediated glucose uptake, that increase risk of coronary heart disease. Increases in carbohydrate intake (with reciprocal decreases in fat content) within the boundaries of menus that can be followed in the free-living state have not been shown to decrease insulin resistance, either directly by enhancing insulin sensitivity or indirectly by producing and maintaining weight loss. However, such diets accentuate the metabolic abnormalities that constitute syndrome X. Because substitution of monounsaturated or polyunsaturated fat, or both, for saturated fat results in the same fall in LDL-cholesterol concentration as seen with low fat/high carbohydrate diets, it is concluded that low fat/high carbohydrate diets should be avoided in the treatment of syndrome X.