The presence of activating mutations of the epidermal growth factor receptor (EGFR)-gene identifies a distinct and clinically relevant molecular subset of non-small-cell lung cancer. It is now well demonstrated that EGFR tyrosine kinase inhibitors (TKIs) gefitinib and erlotinib are superior to standard chemotherapy in this subset of tumors. Nevertheless, in many cases, responses are not durable and last for 6-12 months due to the occurrence of secondary or acquired resistance. Here we present three cases of EGFR-mutant lung adenocarcinomas (ADC), that showed an unexpected response to anti-EGFR small molecules. The first patient presented a continued 89 month-long response to erlotinib in a tumor recurred after surgery and conventional chemotherapy. In the other cases, subclinically persistent tumor in the lung tissue was documented histologically in lung resections performed after partial response to TKI treatment. The persistence of interstitial and endolymphatic tumor cells after TKI treatment might explain the common observation of tumor relapse after TKI discontinuation, and sustain the decision to continue treatment in responsive patients as in our first case.