Background: Neoadjuvant pembrolizumab improved outcome of patients with recurrent Glioblastoma (GBM) in two early phase clinical trials. However, several large phase II/III studies in patients with newly diagnosed and recurrent GBM failed to demonstrate a therapeutic benefit of anti-PD-1 therapy. Therefore, identification of biomarkers of response is crucial for appropriate patient selection and further clinical development of anti-PD-1 therapy. We reported the outcome of our window-of-opportunity clinical trial of neoadjuvant pembrolizumab in 15 patients with recurrent GBM, demonstrating rare CD8+ T cells and abundant of CD68+ macrophages in GBM tissue after 3 weeks of anti-PD-1 treatment (NCT02337686). In the current study, we compared tumor infiltrating lymphocyte (TIL) and PD-L1 scores, known biomarkers of response to anti-PD-1 therapy in other cancers, in pre-trial vs. on-trial tumor tissue and associated these markers with survival. Methods: We determined TIL score (morphological assessment of the presence or absence of TILs, 0-3) and PD-L1 H score (defined as [1*1+ %]+[2*2+ %]+[3*3+ %], 0-200) and correlated these with survival. The Wilcoxon signed rank test was used to compare levels of PD-L1 H or TIL scores between pre-trial and on-trial specimens. The Cox proportional hazards models were used to assess associations between correlative markers and progression free survival or overall survival (OS). Results: The on-trial TIL level (median: 3) was significantly higher than the pre-trial TIL level (median: 1) (p = 0.031). However the difference between pre-trial and on-trial PD-L1 levels was not statistically significant (p > 0.9). Patients whose on-trial PD-L1 H score was ≥ 3 trended toward a longer OS than those with a PD-L1 H score < 3 (HR [95% CI] = 0.225 [0.043, 1.183]) (p = 0.0782). Conclusions: Although GBM tissue lacks abundant T cells, treatment with pembrolizumab increases trafficking of T cells to the tumor microenvironment, which is necessary but not sufficient to induce an effector T-cell response. Elevated PD-L1 expression may be a biomarker of response to anti-PD1 therapy in GBM, which needs confirmation in larger studies. Further genomic, transcriptomic, and methylation profiling of the pre-trial and on-trial tissues is ongoing. Clinical trial information: NCT02337686.