Klinefelter Syndrome is the most frequent cause of hypergonadotropic hypogonadism in men. A flat response at luteinizing hormone releasing hormone stimulation test could be the first sign of hypothalamic tumor in these patients. We report the case of a patient diagnosed by neonatal screening with Klinefelter Syndrome, 47 XXY, that at 17 years follow-up presents analytical modification of the response to luteinizing hormone releasing hormone stimulation test with suppressed luteinizing hormone and follicle-stimulating hormone values; lately he presents with headache and loss of left eye vision. A magnetic resonance imaging of the brain showed a mixed germ cell hypothalamus tumor, requiring surgery, chemotherapy and radiotherapy with optimal response.