Tumor infiltrating lymphocytes have been correlated with a better prognosis for some tumors and medullary carcinoma of breast is a good example. However, in a recent study of invasive micropapillary carcinoma of breast, tumor infiltrating lymphocytes were associated with increased lymph node metastasis and a poorer prognosis. To explore possible mechanisms underlying this difference in immune responsiveness and tumor behavior, 28 cases of invasive micropapillary carcinoma with prominent lymphocyte infiltration were compared with 29 cases of medullary carcinoma. In both tumors, the majority of tumor infiltrating lymphocytes were T lymphocytes (P<0.01) with CD8+ T lymphocytes predominant (P<0.01). Significantly, functional differences in CD8+ cytotoxic T lymphocytes were identified in the two types of tumor. While lymphocytes infiltrated both the stroma and epithelial components of medullary carcinoma, the tumor infiltrating lymphocytes of invasive micropapillary carcinoma were almost exclusively confined to the stroma. Tumor infiltrating lymphocytes of medullary carcinoma showed stronger expression of FasL than those in invasive micropapillary carcinoma (P<0.01) and medullary carcinoma cells exhibited stronger expression of Fas than invasive micropapillary carcinoma cells did (P<0.01). In the subgroups of tumors with strong (++/+++) Fas expression, double immunohistochemistry revealed that most of the tumor infiltrating lymphocytes in medullary carcinoma, particularly those infiltrating the tumor nests, were CD8+ cytotoxic T lymphocytes, but not so in invasive micropapillary carcinoma. Furthermore, upregulated expression of perforin, granzyme B and FasL by cytotoxic T lymphocytes was greater in medullary carcinoma than invasive micropapillary carcinoma (P<0.01, respectively). The results suggest that effective immunity provided by tumor infiltrating lymphocytes varies in different tumors and the relative lack of tumor-killing cytotoxic T lymphocytes in invasive micropapillary carcinoma may explain, in part, the adverse association of tumor infiltrating lymphocytes with the biological behavior of invasive micropapillary carcinoma of breast.