The association of solid tumors with idiopathic thrombocytopenic purpura (ITP) is rare. Before this study, there have been three case reports indicating an association between breast cancer and ITP. We present a 69-year-old man with metastatic breast cancer and progressive thrombocytopenia without any evidence of a leukoerythroblastic picture or etiologies for the thrombocytopenia. Serum platelet antibodies were identified. A bone marrow biopsy showed small foci of metastatic breast cancer, thrombocytopenia, and normal number of megakaryocytes. A subsequent trial of steroids resulted in a marked improvement of the patient's thrombocytopenia. This is the fourth case report indicating an association of breast cancer and ITP.