Medullary carcinoma (MC) of the breast is considered to carry a more favorable prognosis than other subtypes of infiltrating ductal carcinoma. This is a biological paradox because its clinical behavior contrasts with its anaplastic morphology. MC is characterized by a dense lymphocytic infiltrate. In this study, we determined the immunological profile of tumor infiltrating lymphocytes (TILs) in MC by CD20 (L26), CD8, and CD45RO (UCHL 1) immunostaining on paraffin-embedded sections. We examined 14 cases of typical MC (TMC), 15 cases of atypical MC (AMC) classified according to Ridolfi criteria (1977) and 19 cases of poorly differentiated infiltrating ductal carcinoma (PDC-NOS). TILs were quantified separately into cells infiltrating tumor nests (intraepithelial) and cells infiltrating tumor stroma (stromal). The number of CD8 positive and CD20 positive cells infiltrating tumor nests and tumor stroma were significantly increased in TMC and AMC as opposed to the PDC-NOS group. There was a loss in the number of CD45RO positive cells, both intraepithelial and stromal, in TMC and AMC as opposed to the PDC-NOS group. We conclude that MC tumor lymphocytic infiltrate demonstrates a mixed-T cytotoxic (CD8+) and B cell (CD20+)-immunophenotypic profile. This might in part explain the improved clinical outcome of the disease.