The authors describe the case of a 75-year-old female who was hospitalized for anemia of unknown origin. Physical examination revealed a swelling in the right mammary region, where a mastectomy scar was present from surgery for a breast carcinoma. On admission, laboratory tests disclosed anemia (Hb, 8.5 g/dl), with a reticulocyte count of 65,000/mm3 and slightly increased bilirubin. Immunohematologic study revealed the presence of a red cell autoantibody with anti-D specificity in the serum and in the eluate from the patient's erythrocytes. A biopsy of the swelling was performed and histologic examination showed the presence of metastatic cells of breast carcinoma. The patient was given chemotherapy and radiotherapy. At this writing the anemia was absent, the immunohematologic study was negative, the swelling was greatly reduced, and no other metastatic lesions of breast carcinoma were present.