To examine the effects of hnRNP K on the growth and apoptosis of lung adencarcinoma cells in vitro. The expressions of hnRNP K were detected by the SP method of immunohistochemistry in 70 lung adencarcinoma tissue samples. The hnRNP K siRNA was transfected into human lung adencarcinoma cell line A549 with Lipofectamine 2000. The cells transfected with siRNAn and untreated served as controls. The inhibitory effect of siRNA on the expression of hnRNP K mRNA and protein was detected by RT-PCR and Western blot. The change in cell cycling and cell apoptosis of siRNA-treated cells was detected by flow cytometry. Positive expressions of hnRNP K were found in 38.5%, 95.2% and 91.7% of lung adencarcinoma tissues in tumors with a diameter of < or =3 cm, 3-5 cm and > or =5 cm respectively (P < 0.01). The expressions of hnRNP K mRNA were significantly inhibited in siRNA- transfected cells compared with those in controls (P < 0.05). hnRNP K protein decreased in hnRNP K siRNA-transfected cells. siRNA targeting human hnRNP K inhibited the growth of lung adencarcinoma cell line A549 and the distribution of the cell cycle, with an apoptosis rate of 4.79%. Twenty four hours after transfection, the number of cells in G0/G1 phase increased from 37.21% to 85.60%, while the number of cells in S and G2/M phases decreased from 47.71% and 13.00% to 13.50% and 0.32%, respectively. The expression of hnRNP K is closely correlated with the size of lung adencarcinoma. hnRNP K siRNA inhibits the growth aNd increases the apoptosis of lung adencarcinoma cells.