Adenocarcinomas of nonsalivary origin represent approximately 10% to 20% of all sinonasal malignancies and are characterized by varying histopathologic features and uncertain histogenesis. To better understand the histogenesis and phenotypic heterogeneity of these tumors, we performed immunohistochemical analyses for cytokeratin (CK) 7 and CK20 on 12 primary sinonasal adenocarcinomas (SNACs) representing the histopathologic spectrum of these tumors, adjacent normal mucosa, and 2 metastatic adenocarcinomas from colonic primaries. The demographic and clinicopathologic characteristics of our cohort were similar to those in previously published series. Our results indicate that histologically normal respiratory-type epithelium and submucosal seromucous glands show restricted reactivity to CK7. Epithelial metaplasia of surface epithelium associated with enteric SNACs was accompanied by a conversion from CK7 positivity to CK20 positivity. All primary enteric-type carcinomas and the 2 colonic metastases were reactive to CK20, but all nonenteric-type tumors were negative for CK20 (P=0.003) and positive for CK7. In some of the enteric types, coexpression of CK7 and CK20 was noted. We conclude that (1) nonenteric-type (seromucinous) adenocarcinoma may originate directly from surface respiratory-type epithelium or from seromucous glands, (2) metaplastic transformation of surface respiratory to enteric-type epithelium precedes the development of enteric adenocarcinoma, and (3) coordinate analyses of CK7 and CK20 reactivity may aid the differential diagnosis of adenocarcinoma in the sinonasal tract.