The epithelial-mesenchymal transition (EMT) plays a key role in tumor progression, drug resistance and metastasis. Recently, numerous microRNA (miRNA) have been described to regulate EMT in tumor progression. In this study, we found that conditioned medium from the LC212 non-small-cell lung cancer (NSCLC) cell line (LC212-CM) induces morphological changes and overexpression of Vimentin, CD90, SMAD 2/3, SLUG and TWIST in A549 NSCLC cells, consistent with a mesenchymal phenotype. To identify the soluble mediators in LC212-CM involved in this phenomenon, we performed miRNA profiling and TGF-β1 quantification. We found that LC212-CM contains high levels of TGF-β1 as well as different secreted miRNAs. We focused our attention on Homo sapiens-microRNA21 (hsa-miR21), one of most relevant miRNA associated with lung cancer progression, metastasis and EMT. An hsa-miR21 antagomiR was able to prevent the LC212-CM-induced EMT phenotype in A549 cells. Furthermore, we found that TGF-β1 and hsa-miR21 cooperate in the induction of EMT in A549 cells. Intriguingly, TGF-β1 was found to induce hsa-miR21 expression in A549 cell, thus suggesting that the hsa-miR21 mediates at least in part the pro-EMT effects of TGF-β1. In conclusion, hsa-miR21 and TGF-β1 are involved in autocrine and paracrine circuits that regulate the EMT status of lung cancer cells.