An 8-year-old boy presented with precocious puberty and a mediastinal mass. A computer search showed that this rare presentation is most common with germ cell tumor of the mediastinum in children with Klinefelter syndrome. The tumor was completely resected after preoperative chemotherapy, and the patient is well 2 years after the operation. In patients with Klinefelter syndrome, germ cell tumors are 50 times more common than in patients without Klinefelter syndrome, usually contain nonseminomatous elements, present at an earlier age, and are seldom testicular in location.