Previous studies of mast cells in invasive breast carcinoma have suggested that the presence of mast cells in the peritumoral stroma is associated with a favorable prognosis.1–4 It has further been suggested that this association is due to a correlation between mast cell infiltrates and low tumor grade.4 c-kit is a transmembrane tyrosine kinase receptor protein, which is expressed by hematological stem cells, germ cells, melanocytes, and interstitial cells of Cajal, and by both normal and neoplastic mast cells.5–7 We used immunostaining for c-kit (CD-117) to identify mast cells in the stroma of invasive breast carcinomas, using tissue microarrays containing 348 cases with clinical follow-up data, to determine whether the presence of stromal mast cells was of prognostic significance in this large cohort of patients. Immunostains for T- and B-lymphocytes and histiocytes were also assessed on the same tissue microarrays to determine whether there was a correlation between the presence of mast cells and other inflammatory cells. Finally, the relationship between the presence of mast cells and other established prognostic markers, including hormone receptor expression, Her2 status, grade, and lymph node status, was assessed.