BACKGROUND Carcinosarcoma of the sinonasal tract is an extremely rare malignant neoplasm; it is often designated as carcinoma with spindle cell or sarcomatoid features. We report a case of carcinosarcoma arising in a pre-existing inverted Schneiderian papilloma in the left maxillary antrum and nasal cavity of a 72-year old male patient. CASE REPORT The patient had a significant history of radiotherapy for squamous cell carcinoma in the sinonasal area, 3 decades ago. The patient presented with chief complaints of left nasal blockage, nasal discharge, anosmia, and occasional epistaxis. Computed tomography scan displayed a lobular soft tissue mass resulting in narrowing of the nasopharyngeal airway with massive destruction of palatal tissue. The lesion was resected via endoscopic surgery. Macroscopically, a white fleshy appearance with necrosis was noted in the submitted specimen. Microscopically, the tumor was composed of pleomorphic epithelial and spindle cells with numerous mitoses and remarkable tissue necrosis. Residual inverted papilloma (IP) with high-grade dysplasia, and minimal foci of moderately differentiated squamous cell carcinoma (SCC) component was present at the tumor margin. A distinct zone of transition of SCC to spindle cell carcinoma (SpSCC) was noted and confirmed by focal positivity of p63 in epithelial and sacromatoid components. The pleomorphic sarcomatoid tumor was positive for vimentin with Ki67 highlighting 70% of tumor cells. A final diagnosis of sinonasal spindle cell carcinoma associated with residual inverted papilloma was rendered. CONCLUSIONS Due to the rarity of such cases, the prognosis and response to treatment is unclear. No effective directed treatment has been developed. Unfortunately, the patient refused any further treatment and died of persistent disease. To the best of our knowledge, only one case of sinonasal carcinosarcoma arising from dysplastic inverted papilloma has been reported. The distinct possibility of previous radiotherapy contributing to development of sarcomatoid features in this neoplasm should also be considered.