Klinefelter syndrome is the most common (0,1–0,2% male newborns), genetic form of infertility and hypergonadotropic hypogonadism in men. It develops as a result of numerical aberrations of chromosome X (usually 47,XXY). In childhood and early puberty activity of the hypothalamo–pituitary–testicular axis is usually normal. From the middle of puberty (GIII Tanner’s stage) develops the clinical picture of hypergonadotropic hypogonadism due to progressive degeneration of the structure and impaired testicular function. Th e phenotype is variable, ranging from almost normal to signifi cantly diff erent. Newborns with Klinefelter syndrome usually present normal male phenotype. Often the only clinical symptom is small testes, which are usually identifi ed after puberty. Patients with that syndrome are usually infertile, however in about half the cases, it is possible to fi nd spermatozoa in the testes, and have a higher risk of i.e. breast cancer, metabolic syndrome, cardiovascular disease, osteopenia/osteoporosis, autoimmune diseases. In addition, there is varying degree of cognitive, social, behavioral and learning diffi culties. Early diagnosis of Klinefelter syndrome is recommended to implement early therapy and prevention of co -morbidities. key words: numerical aberration of chromosome X, small testes, tall stature, hypergonadotropic hypogonadism, infertility.