Hospitalization of obese patients on a 45% carbohydrate diet resulted in a decrease of insulin binding to circulating monocytes. A further decrease was observed when the carbohydrate content was increased to 75% of total calories. When carbohydrate intake was restricted to 10% of total calories, insulin binding returned to normal. Total caloric intake was kept constant so that changes in insulin binding were not due to changes in body weight. Serum insulin levels were elevated in all obese patients regardless of what diet they were on receiving. The serum glucose level rose slightly in the patients on carbohydrate restriction despite the rise in insulin binding and persistance of hyperinsulinemia. I concluded: 1) factors other than a defect in insulin receptors contribute to insulin resistance in obesity; 2) although high insulin concentrations decrease insulin receptors in cells grown in vitro, cells from patients with hyperinsulinemia do not necessarily show decreased insulin binding; and 3) insulin binding is affected by the carbohydrate content of the diet and probably by other factors as well.