The tissue sensitivity to insulin was evaluated using the hyperinsulinemic euglycemic clamp technique in a 44-year old male with Werner's syndrome associated with diabetes mellitus. In addition, the autophosphorylation of insulin receptors and the number of autophosphorylated insulin receptors were measured in his erythrocytes by a new enzyme-linked immunosorbent assay. He had an increased serum insulin level (30.5 microU/ml) in addition to hyperglycemia (226 mg/dl), suggesting that he had insulin-resistant diabetes mellitus. A clamp study revealed that his insulin-stimulated glucose disposal rate (2.80 mg/kg/min) was comparable to that in noninsulin-dependent diabetes mellitus (4.14 +/- 1.94 (SD) mg/kg/min, n = 23). The number of autophosphorylated insulin receptors per 300 microliters of erythrocytes was also identical to that in normal control subjects. In addition, the autophosphorylation of insulin receptors determined at six different insulin concentrations was within the normal range, even when it was expressed as per 300 microliters of erythrocytes as well as per unit receptor. These results demonstrate that insulin resistance in the patient with Werner's syndrome is caused by a defect that cannot be detected by means employed in this study, and suggest that the defect is present at the steps distal to the autophosphorylation of tyrosine kinase of insulin receptors.