Klinefelter's syndrome and mediastinal germ cell neoplasms.

We describe 2 patients with Klinefelter's syndrome in whom mediastinal germ cell neoplasms developed. This association also has been noted in the literature in 6 other cases. The pathogenesis of these otherwise rare neoplasms probably is related to the genetic abnormality and the abnormal hormonal status in Klinefelter's syndrome. Patients with Klinefelter's syndrome appear to have an increased risk of development of extragonadal mediastinal germ cell neoplasms and should be followed closely, while patients with mediastinal germ cell neoplasms should have karyotypes performed.