The sinonasal tract may give rise to a broad range of neoplasms that share a "small round blue cell" tumor (SBRCT) appearance on routine histology, but treatment strategies depend on precise tumor classification. Immunohistochemistry for p63 is often employed in the sinonasal SRBCT differential diagnosis because it is highly sensitive for squamous cell carcinoma (SCC). However, p63 staining may be observed in other tumor types, a potential diagnostic pitfall. P40 is a more squamous-specific isoform of p63, and it may be more useful in distinguishing poorly differentiated SCC from its mimickers in the sinonasal tract. Immunohistochemistry for p40 and p63 was performed on 171 sinonasal neoplasms with SRBCT morphology: 73 SCCs (67 poorly differentiated, non-keratinizing, or basaloid types and 6 nasopharyngeal carcinomas), 46 esthesioneuroblastomas, 11 sinonasal undifferentiated carcinomas (SNUCs), 11 lymphomas, 9 melanomas, 7 alveolar rhabdomyosarcomas, 4 solid adenoid cystic carcinomas, 4 NUT midline carcinomas, 4 primitive neuroectodermal tumors (PNETs), and 2 small cell carcinomas. P40 was positive in 72 of 73 SCCs, and showed a diffuse distribution in all but one positive case. P40 immunoexpression was also observed in 13 of 46 (28 %) esthesioneuroblastomas, 6 of 11 (55 %) SNUCs, 2 of 4 (50 %) adenoid cystic carcinomas, 3 of 4 (75 %) NUT midline carcinomas, 1 of 2 (50 %) small cell carcinomas, and 1 of 4 (25 %) PNETs; in the non-SCC tumors, p40 staining was focal in most cases. P63 was positive in every p40-positive tumor. In addition, a p63+/p40- phenotype was seen 5 of 11 (45 %) lymphomas, 4 of 7 (57 %) alveolar rhabdomyosarcomas, 1 of 4 (25 %) PNETs, and 3 of 46 (7 %) esthesioneuroblastomas. All sinonasal melanomas were negative for both markers. In the sinonasal SRBCT differential diagnosis, both p40 and p63 are highly sensitive for SCC, but p40 is more specific. Notably, p40 is consistently negative in lymphomas and alveolar rhabdomyosarcomas, two tumors that are frequently p63-positive. It must be remembered, however, that even diffuse p40 immunostaining is not entirely specific for the squamous phenotype, and therefore it should be utilized as part of an immunohistochemical panel.