We report four cases of sinonasal teratocarcinosarcoma (SNTCS), a rare malignant tumor that displays combined features of an immature or malignant teratoma and a carcinosarcoma. The patients, three men and one woman, were all adults ranging in age from 21 to 69 years who presented with nasal obstruction and epistaxis. The tumors were typically composed of round cells and short spindle cells with neuroectodermal/rosette-like structures. Also seen were foci of fetal-like squamous epithelium, glandular epithelium, immature mesenchyme, immature cartilage, and neuronal differentiation. Immunohistochemistry performed in three cases showed expression of vimentin, CD99 (MIC2), and neuron-specific enolase in most cells, and focal expression of cytokeratin, epithelial membrane antigen, alpha fetoprotein, glial fibrillary acidic protein, chromogranin, and synaptophysin. The tumors were consistently negative for beta human chorionic gonadotrophin, neurofilament protein, and leukocyte common antigen. The entities considered in the differential diagnosis were poorly differentiated carcinomas, sarcomas, and olfactory neuroblastoma. We suggest that these neoplasms arise from a primitive cell capable of organized divergent differentiation.