Immune checkpoint inhibitors (ICIs) are clinically used for treating advanced lung cancer, and some patients have achieved complete remission (CR) with ICI therapy in clinical trials. However, reports summarizing the clinical courses of such patients are limited. We report two cases of lung adenocarcinoma in which CR was achieved with first-line pembrolizumab monotherapy, and the therapeutic effect was maintained after treatment completion. Specific patients can achieve CR, even those who do not meet the previously reported predictors of treatment response other than high programmed death-ligand 1 expression. Thus, biomarkers that can accurately predict the clinical efficacy of ICIs are warranted.