The discovery of sensitizing epidermal growth factor receptor (EGFR) mutations as a predictive marker of sensitivity to first-generation EGFR tyrosine kinase inhibitors (TKIs) has dramatically changed the paradigm of care for advanced non-small cell lung cancer (NSCLC) patients. Unfortunately, the majority of patients with EGFR-mutant NSCLC treated with EGFR-TKIs develop acquired resistance within 14-16 months. T790M mutation recently emerged as a major determinant of acquired resistance to gefitinib and erlotinib. Osimertinib (AZD9291) is a novel mono-anilino-pyrimidine third-generation EGFR TKI targeting both sensitizing and T790M EGFR-mutation which showed promising results in T790M-positive NSCLC. Here we report two cases of gefitinib- or erlotinib-pretreated NSCLCs with a T790M mutation-positive (as assessed on plasma through the therascreen EGFR test) disease and untreated, asymptomatic central nervous system metastases that responded to treatment with osimertinib.