We report a case of extraskeletal Ewing's sarcoma arising in the retro
peritoneal cavity. The tumor was excised en bloc with the left kidney
and infiltrated portion of the left ureter. Histological diagnosis was
confirmed by both electron micrograph and immunohistochemical stainin
g using a newly developed mouse monoclonal antibody 5C11, which specif
ically reacts with Ewing's sarcoma. An adjuvant chemotherapy T-11 regi
men has been successfully preventing the recurrence.