Extranodal follicular dendritic cell (FDC) sarcomas are not a common phenomenon. Due to the scarcity of the identified cases reported in the literature, FDC is probably under-recognized and commonly misdiagnosed. The diagnosis of FDC sarcomas is based on node-based spindle cell lesions, and the expression of CD21, CD35 and clusterin. The most commonly involved extranodal sites include the oral cavity, tonsil, gastrointestinal tract and liver. With the aid of immunohistochemical analysis and the two most reliable FDC markers, CD21 and CD35, the diagnostic accuracy has improved. When FDC sarcoma is suspected histologically, immunohistochemical stains for FDC differentiation should be performed to avoid potential misdiagnosis. This case report concerns the evaluation of a 43-year-old male Chinese patient with a large extranodal FDC sarcoma (20×18×9 cm) in the mesentery with elevated serum CA125 (76.9 U/ml). The diagnosis and treatment of this disease are also discussed.