Klinefelter's syndrome is the most common cause of male hypogonadism with an incidence of 1 in 1000 male births. Individuals have an extra X chromosome. Usually, the karyotype is 47, XXY. Accelerated atrophy of germ cells before puberty results in sterility with small, firm testes. Many patients are tall with relatively long legs. Behavioural disorders and delayed speech development are common. Testosterone therapy may be used to improve the development of secondary sexual characteristics.