Non-small cell lung cancer (NSCLC) patients with activating epidermal growth factor receptor (EGFR) mutations benefit from treatment with EGFR-targeted therapy. While first-generation ("reversible") EGFR tyrosine kinase inhibitors (TKIs) are well established in the treatment of these patients, the remarkably lower efficacy of second-generation ("irreversible") EGFR-TKIs after failure of reversible EGFR inhibition is far less understood. Here we describe an EGFR-mutated patient treated sequentially with both reversible (erlotinib) and irreversible (afatinib) EGFR-TKIs monitored by sequential [(18)F]fluorothymidine (FLT-)PET. Our observations confirm the value of molecular imaging for assessment of pharmacodynamics and early prediction of response and relapse in these patients.