Background:Circulating tumour cells (CTC) have a crucial role in metastasis formation and can consistently provide information on patient prognosis. Epithelial-mesenchymal transition (EMT) is considered as an essential process in the metastatic cascade, but there is currently very few data demonstrating directly the existence of the EMT process in CTCs.Methods:CTCs were enriched by blood filtration using ISET (isolation by size of epithelial tumour cells), triply labelled with fluorescent anti-vimentin, anti-pan-keratin antibodies and SYTOX orange nuclear dye, and examined by confocal microscopy in six patients with metastatic non-small cell lung cancer (NSCLC). In parallel, CTCs were morphocytologically identified by an experienced cytopathologist.Results:Isolated or clusters of dual CTCs strongly co-expressing vimentin and keratin were evidenced in all patients (range 5–88/5 ml). CTCs expressing only vimentin were detected in three patients, but were less frequent (range 3–15/5 ml). No CTC expressing only keratin was detected.Conclusion:We showed for the first time the existence of hybrid CTCs with an epithelial/mesenchymal phenotype in patients with NSCLC. Their characterisation should provide further insight on the significance of EMT in CTCs and on the mechanism of metastasis in patients with NSCLC.