Objective: To investigate whether laminin 5 (LN5) might be a predictor in lung cancer patient treated with gefitinib and estimate the underlying mechanisms. Methods: LN5 and epidermal growth factor receptor (EGFR) mRNA expression level were detected in the tumor tissues of lung cancer patients who underwent surgery resection prior to gefitinib treatment. EGFR exon 19 and 21 mutation status was also detected in these specimens. The association between LN5, EGFR mRNA expression level, EGFR mutation and gefitinib treatment response were evaluated. In vitro study were carried by adding exog- enous LN5 and gefitinib to A549 lung cancer cell line, and Western-blotting was performed to investigate the phosphorylation level of EGFR,Ak, and Erk. Results: The disease control rate according to LN5 mRNA level was 52.9% for the below cut- point group, and 17.6% for the above cut-point (P = 0.009). The in vitro study showed that exogenous LN5 can neutralize the inhibition of phosphor-Akt by gefitinib. Conclusion: Patients with lower LN5 mRNA level would likely benefit from gefitinib. In vitro study indicated that the inhibition of Akt induced by gefitinib might be reversed by LN5. These results provide important insights into the molecular mechanisms underlying sensitivity to gefitinib in lung cancer patients. Objective: To investigate whether laminin 5 (LN5) might be a predictor in lung cancer patient treated with gefitinib and estimate the underlying mechanisms. Methods: LN5 and epidermal growth factor receptor (EGFR) mRNA expression level were detected in the tumor tissues of lung cancer Patients who underwent surgery resection prior to gefitinib treatment. EGFR exon 19 and 21 mutation status was also detected in these specimens. The association between LN5, EGFR mRNA expression level, EGFR mutation and gefitinib treatment response were evaluated. Exog- enous LN5 and gefitinib to A549 lung cancer cell line, and Western-blotting was performed to investigate the phosphorylation level of EGFR, Ak, and Erk. Results: The disease control rate according to LN5 mRNA level was 52.9% for the below cut - point group, and 17.6% for the above cut-point (P = 0.009). The in vitro study showed that exogenous LN5 can neutralize the inhibition of phosphor-Akt by gefitini In vitro study indicates that the inhibition of Akt induced by gefitinib might may reversed by LN5. These results provide important insights into the molecular mechanisms underlying sensitivity to gefitinib in lung cancer patients.