Patients with congenital generalized lipodystrophy are extremely insulin resistant. To ascertain whether this resistance is due to an insulin receptor defect, we tested four young patients with congenital generalized lipodystrophy and seven healthy persons of comparable age for the binding of 125I insulin to mononuclear leukocytes isolated from peripheral blood. Mononuclear leukocytes from patients with congenital generalized lipodystrophy bound significantly less insulin than cells from normal subjects (P less than 0.01). When patients with lipodystrophy fasted for 60 hours, the insulin binding increased. Altered insulin receptors may be responsible for the pronounced insulin resistance and the decreased synthesis of triglycerides in congenital generalized lipodystrophy.