A 10-year-old boy with precocious puberty of 2 1/2 years' duration presented with a malignant thoracic teratoma with elevated levels of beta human chorionic gonadotropin (beta-HCG) and alpha fetoprotein (alpha FP). The mediastinal tumor was completely excised and adjuvant chemotherapy commenced. Within 3 weeks of commencing chemotherapy he developed hematologic abnormalities that, with the subsequent clinical illness, led to a diagnosis of malignant histiocytosis. Death occurred a few days after start of therapy for malignant histiocytosis. Cytogenic studies showed the somatic karyotype to be that of Klinefelter's syndrome, while the malignant cells in marrow and lymph node carried an additional marker chromosome. This case, with others reported, suggest that the XXY karyotype may influence the development of mediastinal germ cell tumors, which in themselves appear to be associated with the early onset of a malignancy of the hemopoietic system.