Klinefelter's syndrome and precocious puberty: a harbinger for tumor.

Boys with Klinefelter's syndrome are at an increased risk of precocious puberty. Most cases are either idiopathic or due to a mediastinal tumor. Patients with Klinefelter's syndrome are at a high risk of primary, extragonadal germ cell tumors, which are usually nonseminomatous, but can be a mixed type with seminomatous elements.