More than half of the men with Klinefelter syndrome (KS) consults a urologist, mainly for a decreased penile size or sexual problems, like a decreased libido, erectile dysfunction or hypospermia. These urological signs and symptoms are not the result of the chromosomal aberration, but the consequence of the hypogonadal status. Recent studies with a functional MRI revealed the trophic role of testosterone on the "emotional brain". Furthermore, this hormone influences the erectile function by enhancing endothelial and neurological pathways. In addition, testosterone increases the sperm volume by stimulating the glandular cells of the seminal vesicles and the prostate. Therefore, testosterone replacement therapy should be started as soon as possible in men with KS and documented hypogonadism to prevent or treat these urological symptoms caused by the hypogonadal status.