Ewing's sarcoma (ES) is a highly malignant tumor composed of uniform small round 
cells. Recently, a single biologic entity, Ewing's sarcoma family of tumors 
(ESFT) has been accepted. The entity includes ES, extraskeletal Ewing's sarcoma 
(EES) and primitive neuroectodermal tumor (PNET). ESFT cells have 
immunoreactivity for CD99, an antigen determined by the MIC2 gene. Most ESFT has 
the (11;22) (q24;q12) translocation. The translocation results in the fusion of 
the EWS gene with the transcription factor gene FLI1 which has been considered a 
hallmark of ESFT. We present an extremely unusual case with ESFT in a spinal 
nerve root mimicking a neurogenic dumbbell tumor. A male aged 20 years noticed 
pain in his right buttock. Magnetic resonance imaging (MRI) revealed a mass in 
the right L5/S intervertebral foramen and the lesions in the sacrum. Surgery was 
performed with a presumptive diagnosis of a nerve sheath tumor. At surgery, the 
tumor was located in the right L5 nerve root sleeve. The sacral lesions were 
observed closely. At one month after surgery, radiologically multiple lesions 
were detected in the pelvic bones. Microscopically the lesions from the root and 
ilium were composed of small round cells immunoreactive for CD99. Reverse 
transcription-polymerase chain reaction detected transcripts resulting from the 
fusion of the EWS gene with FLI1 genes in the iliac lesion. Immunoreactivity for 
CD99 and detection of the EWS-FLI1 hybrid transcripts are important for the 
correct diagnosis of ESFT arising in an unusual location.