Background: Patients with EGFR-mutant lung adenocarcinoma develop progression of disease on TKIs therapy after a median of 12 months;this acquired resistance is mainly due to a secondary mutation in EGFR (T790 M) in about 50% of patients, amplification of MET in 15%, PIK3CA mutations in 5%, an unknown mechanism in almost 30% and a SCLC transformation in some pts. Recently, Takezawa and colleagues pointed out that HER2 amplification is a mechanism of acquired resistance to EGFR inhibition in EGFR-mutant lung cancers without EGFR T790M mutation. To aid in identification and treatment of these patients we examined a cohort of patients whose cancers were assessed with tumor biopsies at multiple times before and after their treatment with TKIs. Methods: 41 lung adenocarcinomas pts. (20 male, 21 female, median age 55 years) with EGFR mutations at 19 or 21 exons received TKIs as first line of treatment. 31 pts. (75%) showed a clinical response and relapsed after a mTTP of 12 months. At the time of relapse a new biopsy was performed, histologic samples were reviewed to re-confirm the diagnosis, EGFR, MET and HER-2 amplification were identified by FISH, while EGFR mutations have been tested by DNA sequencing. Results: At the time that drug resistence was acquired all 31 pts. retained their original activating EGFR mutations, 16 pts. developed EGFR T790M resistance mutation with pronunced EGFR amplification in 5, 4 pts. developed MET amplification, 3 pts. were found to have a diagnosis of small cell lung cancer. HER2 amplification was observed in four pts. (13%), with dramatic progression and a median OS of 5 months after treatment with CDDP + pemetrexed. Notably all 4 cases were EGFR T790M negative. Conclusions: Among pts. with acquired resistence to EGFR TKIs the presence of HER2 amplification defines a clinical subset with a more adverse prognosis and rapid progression. Interestingly, recent data suggest that afatinib combined with cetuximab could have promising activity in pts. with acquired resistance due to HER2 amplification.