Primary mediastinal nonseminomatous germ cell tumor associated with Klinefelter's syndrome is a rare disorder. We experienced a case of recurred primary mediastinal nonseminomatous germ cell tumor developed in a 24-year-old patient with Klinefelter's syndrome. The patient had been treated with surgery and combination chemotherapy under the diagnosis of primary mediastinal nonseminomatous germ cell tumor before. A round mass was found on the right lower lung field in the chest X-ray during follow up. The patient was diagnosed as recurred primary nonseminomatous germ cell tumor and Klinefelter's syndrome through tumor markers, peripheral blood karyotyping, and other tests including hormonal assay and was treated with combination chemotherapy and surgery again. When the patient is diagnosed as primary mediastinal nonseminomatous germ cell tumor, Klinefelter's syndrome and hematologic malignancies should be considered to be associated diseases and vice versa.