Breast carcinomas are often infiltrated by inflammatory cells, particularly macrophages and T lymphocytes, but the significance of these cells remains unclear. One possible role of these inflammatory cells is that they represent a cell-mediated immune response against the carcinoma. CD8(+) lymphocytes are a known crucial component of cell-mediated immunity. The purpose of this study was to explore the prognostic value of tumor-infiltrating CD8(+) cytotoxic lymphocytes in breast cancer. Tumor-infiltrating CD8(+) lymphocytes were assessed by immunohistochemical staining of tissue microarray cores from 1,334 unselected breast tumors from patients with long-term follow-up. The number of CD8(+) T cells was counted in tumor nests (intratumoral), in stroma adjacent to tumor cells, and in stroma distant to tumor cells, and their relationship with clinical outcome was determined. The total number of CD8(+) cells was positively correlated with tumor grade (r(s) = 0.20; P < .001) and inversely correlated with patient's age at diagnosis, estrogen receptor-alpha (ER-α), and progesterone receptor (PgR) expression (Mann-Whitney U test, P < .001). The total patient cohort was randomly divided into two separate training and validation sets before performing univariate survival analysis. Total number and distant stromal CD8(+) lymphocytes were associated with better patient survival (P = .041 and P < .001, respectively) in the training set. In multivariate analysis, total CD8(+) T-cell count was an independent prognostic factor in both training and validation sets. These results suggest that tumor-infiltrating CD8(+) T lymphocytes have antitumor activity as judged by their favorable effect on patients' survival and could potentially be exploited in the treatment of breast cancer.