A large pelvic mass in a 39-year-old man.

Schwannomas are encapsulated, usually benign, tumors composed of well-differentiated Schwann cells, arising from the neural sheath of peripheral nerves. The most common type of benign schwannoma is the acoustic neuroma arising from the eighth cranial nerve. Retroperitoneal schwannomas account for 0.5-3% of the retroperitoneal tumors in most series. We present a case of a 39-year-old man with a pelvic mass, arising from a sacral nerve root, diagnosed as a schwannoma.