BACKGROUND/AIM: Immunotherapy using immune checkpoint inhibitors (ICIs) has revolutionized the treatment of advanced non-small cell lung cancer (NSCLC). Although several ICI options are available, the treatment regimen for NSCLC with large size tumors (large NSCLC) is controversial and the efficacy of anti-CTLA-4 antibody is unclear. This study thus investigated potential biomarkers for CTLA-4 blockade. PATIENTS AND METHODS: The correlation between tumor diameter and treatment duration was examined in patients with advanced NSCLC treated with anti-PD-1 antibody monotherapy in our institution. In addition, the ratio of tumor-infiltrating CD8 T cells and regulatory T (Treg) cells in small and large size NSCLC was also evaluated using immunohistochemical staining. Finally, the efficacy of treatment with anti-CTLA-4 antibody against large NSCLC was investigated. RESULTS: A negative correlation was found between tumor diameter and treatment duration in patients treated with anti-PD-1 antibody monotherapy. Immuno-histochemical staining revealed that Treg cell infiltration was significantly higher in large NSCLC tumors than in small tumors. Among the patients with large NSCLC, the ICI regimen including anti-CTLA-4 antibody showed significant efficacies. CONCLUSION: Anti-PD-1 antibody monotherapy might be less effective against large NSCLC due to the infiltration of Treg cells. Therefore, it might be appropriate for large NSCLC to select a treatment including an anti-CTLA-4 antibody, which can target Treg cells.