In conclusion, although our findings must clearly be interpreted with caution due to our small sample size, we have found that the disappearance or persistence of T790M in the serum/plasma in patients with lung adenocarcinoma and EGFR mutations and AR to first- and second-generation TKIs mediated by T790M may be a useful marker of symptomatic and radiographic evolution to osimertinib, a third-generation TKI able to target both EGFR sensitizing and T790M mutations. Here, EGFR is linked to lung adenocarcinoma.