['Distinguishing small cell epithelial malignancies of the sinonasal cavity and nasopharynx is difficult due to overlapping morphologic characteristics, particularly in small biopsies. This distinction is important, however, because of the inherent differences in biology, natural history, prognosis, and treatment among these neoplasms. The aim of this study is to identify a limited immunohistochemical panel that may help to differentiate these morphologically similar small cell epithelial malignancies.', 'We reviewed 37 cases of histologically similar small cell epithelial malignancies of the sinonasal cavity and nasopharynx: nasopharyngeal carcinoma (NPC) (16), basaloid squamous cell carcinoma (BSCC) (15), and high-grade neuroendocrine carcinoma (HGNEC) (6) obtained at Jackson Memorial Hospital/UM Sylvester Comprehensive Cancer Center between 2003 and 2007. Immunohistochemistry for pancytokeratin (CK), CK5/6, p63, and HLA-DR was performed using the labeled streptavidin-biotin method.', 'All cases in this study were positive for CK and p63. The CK staining pattern of HGNEC was characteristically dot-like whereas the remaining tumors stained with strong and diffuse cytoplasmic membrane positivity. Likewise, the p63 staining pattern of HGNEC was focal and weak whereas the remaining tumors stained with diffuse and strong nuclear positivity. Immunohistochemistry for HLA-DR was positive in all cases of NPC, whereas BSCC and HGNEC were uniformly negative. Cases of NPC and BSCC were positive for CK5/6 whereas cases of HGNEC were negative.'] Does a limited immunohistochemical panel help differentiate small cell epithelial malignancies of the sinonasal cavity and nasopharynx?