Klinefetter syndrome is one of the most common genetic causes of male infertility and the most common cause of primary testicular failure. Beside the infertility issue, the syndrome causes decreases in bone mass, muscle wasting, decline in cognitive ability and increases the risk of diseases such as diabetes mellitus, cancer and cardiovascular diseases. Most men are diagnosed late when fertility problems arise and some even remain undiagnosed. It is probably emerging because of the significant differences in clinical appearance and low "awareness" among primary doctors. Early diagnosis and hormone replacement therapy can significantly improve the quality of patients lives, reduce late health complications, and may even preserve their fertility for a longer period of time. In the past, men with this syndrome, especially those of the non-mosaic type, were considered as having no chance of becoming biological fathers. However today, with the technoLogical development of IVF and the ICSI (intracytoplasmatic sperm injectioni, together with testicular biopsies and sperm extractions (TESE), it is possible to help over 50% of the men who have this syndrome.