Objectives: The expression patterns of cytokeratin (CK) filaments in human epithelial neoplasms are complex and distinctive. The aims of this study were to analyze CK expression and to evaluate the diagnostic application of CKs in human non-small cell lung cancer (NSCLC). Methods: mRNA expression of CK5, CK6, CK14, CK15, CK17, and CK19 was analyzed by Northern blotting. Protein expression of CK5/6, CK7, CK14, CK17, and CK18 was evaluated by immunohistochemistry on tissue microarrays. Results: Northern blotting showed that CKs were highly expressed in human bronchial epithelial cells and/or small airway epithelial cells. In NSCLC cell lines, the expression pattern of CKs was heterogeneous. Regarding protein expression of CKs in 95 primary lung tumors, expression of CK5/6, CK14, and CK17 proteins was increased in squamous cell carcinomas compared to adenocarcinomas (ADC; p = 0.001, p = 0.030, and p = 0.001, respectively), and higher expression was significantly associated with lower grading (p = 0.006, p = 0.002, and p = 0.001, respectively), while increased expression of CK7 and CK18 was observed in ADC (p = 0.001, respectively). Conclusions: Our data suggest that CK5/6, CK7, CK14, CK17, and CK18 have diagnostic value in the subclassification of NSCLC. Copyright (C) 2011 S. Karger AG, Basel