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Osimertinib is a third-generation tyrosine kinase inhibitor for non-small cell lung cancer (NSCLC) patients with epidermal growth factor receptor (EGFR)-sensitizing mutations and acquired drug-resistant mutation T790M. Despite promising treatment benefits of osimertinib in first- and second-line settings, drug resistance has been an inevitable clinical issue. The resistance to osimertinib is heterogeneous, which may involve EGFR-dependent and independent mechanisms as well as histological transformation from NSCLC to small cell lung cancer (SCLC). Current clinical studies of NSCLC were mainly focused on patients with EGFR-sensitizing mutations or acquired T790M mutation or both. The treatments and drug-resistant mechanisms in patients with de-novo T790M mutation remain undefined. Herein, we reported the presence of the less common de-novo EGFR T790M mutation in a stage IV NSCLC patient. The patient received osimertinib as first-line treatment and achieved durable progression-free survival (PFS) for 24 months. After osimertinib resistance, tumor biopsy indicated histologic transformation from NSCLC to SCLC. Given persistent presence of de-novo T790M mutation, osimertinib was used in combination with etoposide and cisplatin as second-line treatment and the patient achieved partial response with PFS of 7 months. Our study suggested that NSCLC patients with de-novo T790M mutation could also benefit from osimertinib and the SCLC transformation may be a potential resistance mechanism that could be targeted through the combination of targeted therapy and chemotherapy.