Klinefelter syndrome is one of the most common forms of primary hypogonadism and infertility in males. It is a clinical syndrome consisting of gynecomastia, azoospermia, and increased urinary excretion of follicle-stimulating hormone. Fequency of diabetes mellitus, emphysema, asthma, breast cancer increase in Klinefelter syndrome. We report a 16-year-old male patient with impaired glucose tolerance in association with Klinefelter syndrome, which was confirmed by chromosome analysis. The mechanism of impaired glucose tolerance in this patient was peripheral insulin resistance which clarified by euglycemic hyperinsulinemic clamp test.