Ten cases of an unusual form of carcinoma involving the mucosa and underlying tissue of the tongue, hypopharynx, and larynx are described. All ten of the tumors were evaluated by light microscopy; five were also studied by electron microscopy. The major histopathologic feature is carcinoma with a basaloid pattern in intimate association with squamous cell carcinoma, carcinoma in situ, or focal squamous differentiation. The basaloid tumor consists of small crowded cells with hyperchromatic nuclei, scant cytoplasm, small cystic spaces, and foci of tumor necrosis. Prominent hyalinosis is evident. Ultrastructurally, the basaloid epithelial cells possess rare tonofilaments and varying amounts of desmosomes. The cystic spaces contain either loose stellate granules or replicated basal lamina arranged in parallel stacks or globoid masses. This unique tumor was found to be highly malignant, with histologically proved metastases in 80 per cent of the cases. Most of the patients were treated by radical surgery supplemented with radiation and/or chemotherapy. It is concluded that tumors with these characteristic features constitute a distinct histopathologic entity, not previously described, for which basaloid-squamous carcinoma is an appropriate term.