Klinefelter's syndrome is characterized by small testes, azoospermia, gynecomastia, and elevated levels of plasma gonadotropins in men with two or more X chromosomes. Previous investigators reported that patients with Klinefelter's syndrome are predisposed to the development of a non-seminomatous germ cell tumor in the mediastinum. It is suggested that this linkage may be due to the hormonal imbalance in Klinefelter's syndrome and consequently, the formation of dysgenetic germ cell and/or abnomal migration of germ cell. We report here a case of Klinefelter's syndrome in a 24-years-old man who was presented with anterior mediastinal mass. The clinical and laborarotory findings were compatible with Klinefelter's syndrome and he was found to have 47 XXY karyotype. Pathological findings for mediastinal mass revealed mixed germ cell tumor composed of mature cystic teratoma and endodermal sinus tumor. He was treated with cis-platin containing chemotherapy and followed up in partial remission.