ABSTRACT Aim: To investigate TIL infiltration and the PD1/PD-L1 axis in NSCLC BM. Methods: We performed immunohistochemistry for CD3, CD8, CD45RO, PD1 and PDL1 in 73 NSCLC BM specimens and 25/73 (34%) matched PT specimens of 73 patients (46/73 63%) male; 27/73 (37%) female; 15/73 (21%) squamous histology; 58/73 (79%) non squamous histology). Results: BM and PT both showed accumulation of TILs in the tumor stroma, perivascular area and at the tumor border to the surrounding parenchyma. Strong, membranous PDL1 expression was observed in 25/46 (52%; 12/46 (26%) expression in >5% of tumor cells) BM and in 8/25 (32%; 3/25 (12%) expression in >5% of tumor cells) matched PT. Accentuation of PDL1+ tumor cells at the tumor border and in regions with high TIL density was evident in BM and PT. Significant correlation of infiltration density between BM and matched PT was observed for CD3+ TILs (p = 0.002) and CD8+ TILs (p = 0.003). Dense infiltration with CD3 + , CD8+ CD45R0+ and PD1+ TILs was more frequently observed in PT than in BM, while PD-L1 expression was more frequently observed in BM than in PT. No correlation of survival from diagnosis of BM and TIL infiltration or PDL1 expression was observed (p > 0.05). Conclusions: (1) NSCLC BM contain considerable TILS infiltrates and show high PD-L1 expression. (2) The composition of the immune response differ between tumor sites with BM showing less dense TIL infiltration but more PD-L1 expression than PT. Our data may be relevant for clinical trials with immune checkpoint inhibitors in NSCLC BM. Disclosure: All authors have declared no conflicts of interest.