Tissue microarrays containing 348 cases of invasive breast carcinoma were studied by immunohistochemical staining for CD-117, CD-3, CD-20, CD-68, Her2, estrogen receptor protein, and progesterone receptor protein, and results were correlated with patient outcome. Hormone receptor status (both estrogen receptor and progesterone receptor) correlated with a good outcome while Her2 overexpression was associated with a poor outcome. The presence of mast cells in the stroma, as demonstrated by positive c-kit (CD-117) staining, correlated with a good prognosis (P=0.0036). On subset analysis, this association between the presence of mast cells and favorable prognosis was present in the node-negative patients (P=0.018). The presence of mast cells showed an inverse correlation with the presence of CD-68 positive macrophages. No correlation was observed between the presence of mast cells and either B-cells (CD20-positive) or T-cells (CD3-positive). The presence of stromal mast cells was of prognostic significance independent of nodal status and tumor size (P=0.02). When the multivariate analysis was expanded to include tumor grade, estrogen receptor status and Her2 status, as well as tumor size and nodal status, the presence of stromal mast cells approached significance as an independent prognostic indicator.