BACKGROUND While germ cell tumors generally occur in the gonads, they may also appear at other sites, from the sacrococcygeum area to the central nervous system. This report describes a case of such a tumor in the posterior mediastinum that developed intraspinally in a dumb-bell fashion. METHODS A 2 1/2 year-old girl was admitted for abdominal tenderness, gait disturbance and fever. Clinical examination showed spastic paraparesis and bladder dysfunction. Thoracic X-rays showed a left postero-superior mediastinal mass with rib erosion. MRI showed that this mass had developed intraspinally between the intervertebral foramina and caused spinal cord compression at T4, T5, T6. The tumor was not calcified. Surgical resection via laminectomy was performed in emergency, but the T5 root had to be excised. Pathologic examination showed histologic features of yolk sac carcinoma; the serum alpha-foetoprotein was elevated (12, 400 IU/ml). The patient was given chemotherapy for 6 months and is well 2 years later. CONCLUSIONS Germ cell tumors may appear in unusual sites. They can be identified by measuring biological markers, and this avoid primary surgery.