Klinefelter syndrome (KS) is the most frequent form of male hypogonadism. Still, at least 50% ofcases are not diagnosed, partly because of the great variation in the clinical symptoms, partly because physicians are unfamiliar with KS. Timely treatment with testosterone can contribute to a more positive body image and, consequently, to a healthier psychosexual development in men with KS. Men with KS experience significantly more health problems and an increased risk ofa malignancy. New reproductive techniques no longer automatically mean that men with KS will remain infertile. Treatment with testosterone is in principle life-long to prevent osteoporosis and loss of muscle mass and strength.