Osimertinib is an irreversible tyrosine kinase inhibitor approved for the treatment of metastatic epidermal growth factor receptor (EGFR)-mutated non-small cell lung cancer (NSCLC). In clinical trials, osimertinib has exhibited excellent activity and less toxicity compared to gefitinib, erlotinib and standard chemotherapy.Herein, we describe the case of a 69-year-old man who received first-line osimertinib for metastatic EGFR-mutated NSCLC. Suspected osimertinib-induced pancytopenia together with a partial treatment response was assessed after 10 days of therapy. Osimertinib was resumed at 40 mg daily a month later while the patient exhibited durable stable disease. No other adverse events occurred.In the current case, first-line treatment with osimertinib at 80 mg daily in a patient with EGFR-mutated NSCLC resulted in severe pancytopenia and a rapid treatment response. Dose reduction to 40 mg daily resulted in excellent activity without any further adverse events. Osimertinib could be safely resumed at a reduced dose even after pancytopenia.