Klinefelter's syndrome (KS) is associated with hypergonadotrophic hypogonadism in adulthood. However, limited information exists about the age at which hypogonadism occurs. The hypothalamic-pituitary-gonadal (HPG) axis is transiently activated during the first months of life, offering the opportunity to study testicular function by spontaneous, basal hormone levels. The aim of this study was to evaluate the HPG axis in KS infants.