OBJECTIVE: To evaluate the expression of programmed death 1 ligand 1 (PD-L1) in the cancer tissues and tumor-adjacent normal tissues of patients with invasive ductal carcinoma of the breast and to analyze the relationship between the expression of PD-L1 and the clinicopathological features of patients. MATERIALS AND METHODS: This study included 112 cases of patients with invasive ductal carcinoma of breast who received surgical treatment from March 2012 to February 2016 in Xuzhou Cancer Hospital. The clinical materials of included patients were retrospectively analyzed. The immunohistochemical assay and real-time polymerase chain reaction (PCR) assay were applied to examine the expression of mRNA and protein of PD-L1 in breast cancer specimens of 112 cases and paired tumor-adjacent tissue specimens of 57 cases. The relationship between PD-L1 protein expression and clinicopathological features of patients was analyzed. RESULTS: PD-L1 protein was mainly expressed in the cytoplasm. The positive rate of PD-L1 expression in invasive ductal carcinoma was 19.6% (22/112), and the positive rate of PD-L1 expression of tumor-adjacent normal tissues was 3.5% (2/57), indicating that the positive rate of PD-L1 expression of cancerous tissues was significantly higher than that of in tumor-adjacent normal tissues (P < 0.05); the positive expression of PD-L1 was not related with the patients' age, menopause history, family history of breast cancer, tumor size, and location of the tumor (P > 0.05) while it was related with lymph node metastasis, the clinic staging, and histopathological grading (P < 0.05). Real-time PCR was applied to detect the mRNA expression of PD-L1 in breast-invasive ductal carcinoma with the mean ΔCt value of 7.79 ± 2.25. However, the mRNA expression of PD-L1 in normal tumor-adjacent tissues was of low expression with the mean ΔCt value of 12.37 ± 3.33. The difference was statistically significant (P< 0.05). CONCLUSION: The expression of PD-L1 in breast-invasive ductal carcinoma was significantly increased, and it was related to histological grading, clinical staging, and lymph node metastasis of breast cancer. PD-L1 may be a significant marker for the prognosis of breast cancer patients.