SummaryThe sinonasal undifferentiated carcinoma (SNUC) is an aggressive and rare neoplasm arising in the nasal cavity and the paranasal sinuses. To date, over 50 cases of histologically proven SNUCs have been reported since its original description in 1986. Presenting symptoms include facial pain, nasal obstruction, diplopia, epistaxis, proptosis, and periorbital swelling. The histologic features of this neoplasm include cohesive cells arranged in nests, ribbons, and trabeculae. The cells exhibit hyperchromatic nuclei and a high nuclear to cytoplasmic ratio. A brisk mitotic rate, tumor necrosis, and vascular invasion are prominent features. Confirming the diagnosis of SNUC at the light microscopic level can be challenging, since the microscopic differential diagnosis includes olfactory neuroblastoma, rhabdomyosarcoma, undifferentiated nasopharyngeal carcinoma (lymphoepithelioma), malignant lymphoma, malignant melanoma, and neuroendocrine (small cell undifferentiated; oat cell) carcinoma. Sinonasal undifferentiated carcinoma can be differentiated from these other neoplasms by correlating clinical, light microscopic, histochemical, immunohistochemical, and ultrastructural characteristics. Aggressive, multimodal therapy can provide the best opportunity for local control of this neoplastic process, but the optimal treatment has yet to be determined.