The first case of Klinefelter's syndrome accompanied by diabetes insipidus and diabetes mellitus is reported. A 41-year-old man admitted for hyperosmolar diabetic coma with a past history of diabetes insipidus was diagnosed as having Klinefelter's syndrome by endocrinological examination and sex chromosome analysis. In this case, glucose tolerance test was normalized half a year later and blood glucose was well controlled with diet therapy alone.