Aims Immunohistochemistry is a highly valuable and widely used tool in the subtyping of lung carcinomas. The aim of this study was to identify markers for the differential diagnosis of non‐small‐cell carcinomas. Methods and results We report on the immunohistochemical localization of plakophilin‐1 (PKP1), keratin‐15 (KRT15) and desmoglein‐3 (DSG3) intercellular adhesion proteins in samples from 75 primary non‐small‐cell lung cancers in non‐treated patients. The staining pattern of these proteins differed between squamous cell carcinomas and adenocarcinomas, with no membrane staining in the latter. Membrane staining for all three proteins was characteristic of squamous cell carcinomas. We observed a relationship between the presence/absence of these proteins in the membranes of squamous cell carcinomas and the differentiation grade, with more intense staining in better differentiated areas. Conclusions Staining for these proteins marked intercellular junctions that are characteristic of stratified squamous epithelium and of neoplasias with this type of differentiation, and can be useful in the diagnosis of patients with squamous cell carcinoma of the lung. The high specificity of membrane staining for PKP1 and DSG3 and high sensitivity of cytoplasmic and membrane staining for KRT15 for the diagnosis of squamous cell carcinoma may be useful for the differential diagnosis of non‐small‐cell carcinomas.