Immune thrombocytopenic purpura (ITP) and breast cancer are common disorders. Only six cases in which patients have had both diseases have been reported. We describe a 40-year-old woman who had ITP while responding to therapy for metastatic breast cancer. Given the few reported cases, the diverse presentations of thrombocytopenia during the course of each patient's breast cancer and the variable therapeutic responses of ITP, the association of breast cancer and ITP is probably coincidental.