Data on the clinical features of the Werner syndrome in 102 patients in Japan were collected by sending questionnaires to major hospitals and analyzed. The male-to-female ratio was 3 to 2 and the incidences of consanguinity and familial occurrence were 51% and 39.4%, respectively. These patients were divided into 3 subgroups; group 1, 2, and 3 lacked short stature, cataract, and hypogonadism, respectively. Each group had somewhat different clinical features. Endocrine and metabolic abnormalities in the Werner syndrome patients were compared with those in normal aged subjects. Impaired plasma growth-hormone responses to insulin and arginine were more common and impaired plasma thyrotropin responses to TRH were less common in the Werner syndrome patients than in aged subjects. Plasma LH and FSH levels were higher in most patients than those in age- and sex-matched controls; also, their serum testosterone concentrations were lower than those in age-matched controls and testicular biopsy revealed more marked atrophy than in aged subjects. Serum triiodothyronine levels tended to be lower than in age-matched controls. Oral glucose tolerance test revealed diabetic glucose tolerance in 55% and impaired glucose tolerance in 22%, although fasting blood glucose levels were elevated only in 20%. Plasma insulin response to glucose was more exaggerated in those with the Werner syndrome than in normal aged subjects. The euglycemic glucose clamp method revealed lower glucose disposal rates and insulin sensitivity indices in the Werner syndrome than in normal subjects of similar age. The number of erythrocyte insulin-binding sites was normal in the Werner syndrome patients. These results suggest a postreceptor defect in insulin resistance in the Werner syndrome.