PURPOSE: The purpose of the study is to investigate the prognostic significance of programmed death ligand-1 (PD-L1) expression and tumor-infiltrating lymphocytes (TILs) in HER2+ breast cancer (BC). METHODS: HER2+ BC cases (n  = 191) were collected between 1996 and 2013. Tissue microarray (TMA) slides were stained with two clones of PD-L1 antibodies (28-8 and 22C3) and the percentage of positive membranous staining was scored. TILs of the full sections were also scored using percentage scale. RESULTS: Clone 28-8 had expression in ≥ 1% of the tumor cells in 25.7% of the cases, while clone 22C3 in ≥ 1% of the tumor cells was expressed in 11.5% of the cases. In the multivariate analysis, higher expression of PD-L1 (clone 28-8) in tumor correlated with lower risk of tumor recurrence, with HR of 0.4 (p = 0.033). Higher level of TILs (> 15%) predicts better overall survival (OS) in all patients with HR of 0.35 (p  = 0.0046). In the group of patients who were treated with trastuzumab-based adjuvant chemotherapy, lower PD-L1 (clone 28-8) expression in TILs correlated with tumor recurrence (p  = 0.034). In the group of patients who were treated with non-trastuzumab-based adjuvant chemotherapy, lower TILs and lower PD-L1 (clone 28-8) expression in tumor had borderline statistical significance in association with tumor recurrence (p  = 0.064 and 0.083, respectively). In the group of patients who were treated with trastuzumab-based adjuvant chemotherapy, PD-L1 or TILs was not statistically significant to predict 5-year survival. In the group of patients who were treated with non-trastuzumab-based adjuvant chemotherapy, low TILs (p = 0.009) correlated with 5-year death due to disease. CONCLUSION: We conclude that PD-L1 may have prognostic significance in HER2+ BCs.