EGFR mutant lung cancer responds to EGFR tyrosine kinase inhibitors (TKIs), but all patients eventually develop resistance to EGFR-TKIs. Herein we report a case of EGFR mutant lung adenocarcinoma that acquired resistance to EGFR-TKI with MET amplification and epithelial-to-mesenchymal transition (EMT). A 73-year-old woman was diagnosed with adenocarcinoma harboring an EGFR exon 19 deletion. She received gefitinib as second-line therapy. Tumors were reduced 1 month after gefitinib therapy. However, only a few months later, chest computed tomography results indicated cancer progression. Gefitinib therapy was stopped and docetaxel therapy started. However, she died 13 days after admission. Microscopic examination of postmortem specimens revealed a diffuse proliferation of atypical giant cells in primary and metastatic lesions, but no adenocarcinomatous components as in the antemortem specimens. Immunohistochemical analyses showed that antemortem tumor specimens were positive for CDH1 but negative for VIM. In contrast, postmortem tumor specimens were positive for VIM but negative for CDH1. Genetic analyses revealed MET amplification. We concluded that resistance to EGFR-TKI might be caused by MET amplification and EMT. To our knowledge, no clinical studies have reported that MET amplification and EMT together may be associated with acquired resistance to EGFR-TKI. Second biopsy after the development of EGFR-TKI resistance may be recommended to determine the best therapeutic strategy.