Teratoma is the most common germ cell tumor, which can be divided into the mature and the immature histologically. Concurrent Klinefelter's syndrome may be overlooked in a patient with a germ cell tumor. This is because the tumor that secrets alpha-fetoprotein and beta human chorionic gonadotropin can mimic puberty in a patient with Klinefelter's syndrome, masking the usual clinical signs. In reviewing the literature on the subject, the role of neoadjuvant and adjuvant chemotherapy remains ill-defined for the immature teratoma. Age-dependent prognosis seems to demonstrate that children with immature teratomas have a better outcome. We share the experience of treating a child with immature teratoma with surgical excision alone, and it ended in a local recurrence.