Twenty-four cases of breast cancer and acute leukemia and three cases o f breast cancer and chronic myelocytic leukemia are reported. An additional 54 cases of acute leukemia and 13 cases of chronic myelocytic leukemia associated with breast cancer from the literature are reviewed.The mean interval between the diagnosis of breast cancer and the occurrence of acute leukemia is 6.9 years. In eight patients, the two diseases occurred simultaneously or within one year of each other. Four patients had acute lymphoblastic leukemia; the remainder had acute myelocytic leukemia or one of its variants.Seven patients received no postmastectomy radiotherapy or chemotherapy and the occurrence of acute leukemia between 1 and 23 years later cannot, therefore, be attributed t o the therapy given for the breast cancer. Forty-one patients received postoperative radiotherapy t o the mastectomy site. Combination chemotherapy was administered t o eight patients, five of whom were also treated with radiotherapy.Acute leukemia is estimated t o occur in patients treated for breast cancer with a sevenfold increased frequency over the expected number. This may be due to an increased risk of a second neoplasm in patients with a primary tumor; alterqatively, the qcute leukemia may be related t o the radiotherapy and/or cheqptherapy administered t o treat the breast cancer. Late death from leukemia after chemotherapeutic or radiotherapeutic remission of metastatic breast cancer is preferable t o morbidity and/or early death from inadequately treated breast cancef,