Abstract Purpose: To evaluate the expression of tumor PD-L1 and changes in tumor-infiltrating lymphocyte (TIL) populations in patients with metastatic melanoma treated with targeted MAPK inhibitors. Experimental Design: Ninety-three tumors were analyzed from 40 patients treated with a BRAF inhibitor alone (BRAFi; n = 28) or combination of BRAF and MEK inhibitors (Combi; n = 12). Tumors were excised before treatment (PRE), early during treatment (EDT), and at progression (PROG). Immunohistochemical staining was performed for CD4, CD8, CD68, FOXP3, LAG3, PD-1, and PD-L1 and correlated with clinical outcome. Results: Patients' tumors that were PD-L1 positive at baseline showed a significant decrease in PD-L1 expression at PROG (P = 0.028), whereas patients' tumors that were PD-L1 negative at baseline showed a significant increase in PD-L1 expression at PROG (P = 0.008) irrespective of treatment with BRAFi or Combi. Overall PD-L1 expression highly correlated with TIL immune markers. BRAFi-treated patients showed significant increases in CD4+, CD8+, and PD-1+ lymphocytes from PRE to EDT (P = 0.001, P = 0.001, P = 0.017, respectively), and Combi-treated patients showed similar increases in CD4+ and CD8+ lymphocytes from PRE to EDT (P = 0.017, P = 0.021). Conclusions: The addition of MEKi to BRAFi did not result in significant reduction in immune infiltration in EDT biopsies. This provides support for conducting trials that combine MAPKi with immune checkpoint inhibitors in the hope of improving complete and durable response rates. PD-L1 expression at PROG on MAPK inhibitors varied according to baseline expression suggesting that combining MAPKi with immunotherapies concurrently may be more effective in patients with PD-L1 expression and TILs in baseline melanoma samples. Clin Cancer Res; 21(14); 3140–8. ©2015 AACR. See related commentary by Cooper et al., p. 3102