Despite the demonstrated benefit from epidermal growth factor receptor (EGFR)-tyrosine kinase inhibitor (TKI) based therapies, EGFR mutant lung adenocarcinoma will eventually acquire drug resistance. Transformation to small-cell lung cancer (SCLC) is considered to be a rare resistance mechanism of EGFR-TKI therapy.We describe a case of a 46-year-old man presenting with refractory cough. Percutaneous transthoracic biopsy was performed and confirmed an EGFR exon 21 L858R lung adenocarcinoma. However, the patient relapsed after successful treatment with gefitinib for 1 year, at which point rebiopsy identified an SCLC and chemotherapy composed of platinum and pemetrexed was started. However, despite the brief success of chemotherapy, our patient died of aggressive cancer progression and complications of chemotherapy.Our case highlights the importance of rebiopsy when managing drug resistance and presents a possible origin of the transformed cells. We also summarize the clinical characteristics of cases involving transformed SCLC from previous studies and discuss whether it could be a new subtype of SCLC.