Background: The solitary fi brous tumor is a spindle-cell tumor that very rarely  involves the orbit. The real incidence of solitary fi brous tumor of the orbit is unknown, but it should be considered in the differential diagnosis of any orbital tumor.
Case Report: A 40-year-old woman had a 4-month history of progressive proptosis of the right eye after a blunt trauma to orbit. Computed tomography scan of orbit showed a well circumscribed, non-enhanced homogeneous orbital mass affecting the retrobulbar aspect of the orbit. A lateral orbitotomy approach was performed and the lesions were excised. The histological evaluation showed the tumors were composed of spindle-shaped cells within collagen bundles and vascular channels. Immunohistochemical staining was positive for cluster of differentiation 34 (CD34) and vimentin but was negative for S-100 protein, epithelial membrane antigen, cytokeratin, smooth muscle actin and desmin.
Conclusions: Before 1994 the diagnosis orbital solitary fi brous tumor was confused with other benign orbital tumor. It must be immunohistochemically differentiated from other spindle-cell tumors of the orbit. The treatment is a complete surgical excision, and long-term followup is necessary.

Keywords: solitary fibrous tumor, Immunohistochemical study, Orbit, CD34