Two women are described in whom, on the basis of prior therapy for breast cancer and the presence of painful, lytic bone lesions, an initial diagnosis of metastatic breast cancer was made. Further evaluation established the diagnosis of multiple myeloma in both patients. Neither had evidence of recurrent breast cancer. These cases indicate that women with a history of breast cancer in whom lytic bone lesions develop without evidence of extraskeletal metastases should have the diagnosis of multiple myeloma excluded.