To examine the relationship between cytotoxic T lymphocyte antigen 4 (CTLA-4) expression and breast cancer prognosis, CTLA-4 expression was immunohistochemically detected in paraffin-embedded specimens of primary tumors from 130 patients with breast cancer who had a mean follow-up period of 112 months. CTLA-4 expressed in cytoplasm of breast cancer cells and in cytoplasm and cell membranes of interstitial lymphocytes. Univariate analysis (log-rank) associated higher density of interstitial CTLA-4(+) lymphocytes with longer DFS and OS, but higher tumor CTLA-4 expression with shorter OS. After controlling for age, clinical stage, Scarff-Bloom-Richardson grade, tumor thrombus, ER, PR, HER2 and Ki-67, multivariate analysis (Cox) showed that density of interstitial CTLA-4(+) lymphocytes independently predicted longer DFS (HR 0.315, P = 0.002) and OS (HR 0.313, P = 0.005), whereas tumor CTLA-4 expression independently predicted shorter DFS (HR 2.176, P = 0.029) and OS (HR 2.820, P = 0.007), i.e., patients with high CTLA-4(+) lymphocyte density and CTLA-4(low) tumor cells had the best prognoses. These results indicated that CTLA-4 expression in lymphocytes was associated with better prognosis, but that in tumor cells was associated with worse prognosis. Patients' CTLA-4 profiles might thus be used to predict the benefits and toxicity of CTLA-4 blockade.