Lung cancer classification in small-cell and non-small-cell types was recently challenged by data on the differential efficacy of new cytotoxic agents in specific histotypes. An accurate histotype definition has therefore gained interest in both preoperative and surgical materials, but is a hard task especially in undifferentiated large-cell tumors lacking morphological signs of squamous or glandular differentiation. The responsiveness of these latter subtypes to new drugs apparently more selective for adenocarcinomas or squamous carcinomas is not fully understood, also due to the heterogeneity of diagnostic criteria for this tumor entity. Current immunohistochemical markers are not fully specific and new molecules are to be explored. On the basis of gene expression profiling data, reporting a remarkable differential expression of desmocollin-3 (a protein localized in desmosomal junctions of stratified epithelial) between adeno- and squamous cancers, we immunostained 62 cases of resected undifferentiated large-cell lung carcinomas for desmocollin-3 (and for TTF-1, p63 and mucin stain), to test its ability to identify a (residual) squamous phenotype, if present. Desmocollin-3 was expressed in almost half of the undifferentiated large-cell cancers and was mutually exclusive with TTF-1 (positive in 39%; the remaining 18 % of cases was double negative). Special large-cell carcinoma variants expressed desmocollin-3 in 6 of 6 basaloid, 7 of 12 clear-cell types, again mutually exclusive with TTF-1 expression. None of seven sarcomatoid carcinomas reacted for either marker. In 31 cytological samples diagnosed as 'non-small-cell lung carcinoma', desmocollin-3 was again mutually exclusive with TTF-1 and stained all squamous carcinomas, 1 of 19 adenocarcinoma only, and 50% of large-cell carcinoma (all histologically confirmed). This combined morphophenotypic approach may represent a valid adjunct (for both surgical and cytological samples) in the selection of patients with lung cancer to medical treatments tailored according to different efficacy in different lung carcinomas of the squamous, adeno- and large-cell types.