Retroperitoneal ancient schwannomas are rare tumors, more usually found in the head, neck and flexor surfaces of the extremities. Ancient schwannomas are a subtype of classic schwannomas with a predominance of degenerative changes, calcifications, hemosiderin deposition, interstitial fibrosis and vascular hyaline degeneration. A 33-year-old male was referred on our hospital with a painful mass in left iliac fossa. The patient underwent surgery and intra-operatively the cystic encapsulated mass was found to be retroperitoneal, between the left psoas major muscle and left iliac muscle. On microscopic examination, we found the presence of Schwann cells in regions with high and low cellularity (Antoni A and B areas) and S100 protein immunohistochemical examination was intensely positive, being consistent with the diagnosis of schwannoma. Complete excision is the only method of the surgical treatment; schwannomas are not sensitive to radiotherapy and chemotherapy. Some authors consider that a complete excision of the tumor, while others believe that enucleated or partial excision of the tumor is sufficient. The prognosis is good, and the most common complication is recurrence, possibly by incomplete excision of it being reported in 5-10% of cases. In conclusion, retroperitoneal schwannomas is usually identified incidentally on tomographic images. Diagnosis is based on histopathological examination after surgery and immunohistochemical examination.