Sinonasal undifferentiated carcinoma (SNUC) is a highly aggressive malignant neoplasm that is often difficult to distinguish from other poorly differentiated carcinomas arising in the sinonasal tract. To search for a differential cytokeratin (CK) expression that could be useful for diagnostic purposes, we compared the expression of a large panel of CKs in a series of 6 SNUCs, 10 poorly differentiated squamous cell carcinomas (SCCs), 10 nonkeratinizing squamous cell carcinomas (NKSCCs), and 5 nasopharyngeal-type undifferentiated carcinomas (NPTCs). SCC, NKSCC, and NPTC frequently showed immunoreactivity for CK5/CK6, CK8, CK13, and CK19. In addition, SCC and NKSCC expressed CK14, which was not detected in NPTC, and SCC expressed CK7 (60% of cases) and CK4 (30% of cases), which were absent in NKSCC and NPTC. Three NKSCCs were associated with a Schneiderian papilloma, and the results of the immunostaining were similar in the two components, with the exception of CK4 and CK7, which were expressed by the papilloma and not by the carcinoma. In contrast to other carcinomas, SNUC was characterized by the exclusive expression of CKs of simple epithelia, such as CK8 (100% of cases), CK7 (50% of cases), and CK19 (50% of cases). Thus, there are significant differences in the pattern of CK expression between SNUC, SCC, NKSCC, and NPTC, which could be of diagnostic aid. Moreover, these findings support the hypothesis that SNUC is a separate entity from SCC and NPTC of the sinonasal tract.