Basaloid squamous cell carcinoma (BSCC) is a recently recognized high-grade tumor with a propensity for nodal as well as systemic metastasis and can arise from different anatomic locations. The differential diagnosis includes adenoid cystic carcinoma, small cell neuroendocrine carcinoma and squamous cell carcinoma. Monoclonal antibodies reactive with cytokeratin (34betaE12, AE3, pancytokeratin), as well as other cellular antigens (vimentin [VIM]; synaptophysin [SYNF]; chromogranin A [ChA]; neuron-specific enolase [NSE]; S-100, desmin, smooth-muscle actin [SMA]), were used in an immunoperoxidase method with paraffin-embedded tissue to phenotypically characterize a case with features of BSCC arising in the maxillary sinus. Neoplastic cells reacted with the high-molecular-weight cytokeratin antibody 34betaE12, as well as with other antikeratin antibodies, but failed to react with the antibodies VIM, desmin and SMA and showed variable immunoreactivity for NSE, SYNF and S-100. The staining pattern for NSE was diffuse and intense and reactivity for ChA was inconsistent.