Two cases of acute leukemia in patients with breast cancer are reported. In the first patient, erythroleukemia occurred three years after breast cancer was treated by mastectomy, followed by local radiotherapy ; complementary chemotherapy (melphalan) has been given for twenty-six months. The second patient had onset of acute lymphoblastic leukemia four months after breast cancer was treated by surgery only. This patient subsequently has complete remission. Both patients died shortly after onset of leukemia. In the first patient, bone marrow cytogenetic studies evidenced major abnormalities at an early stage of the disease, with abnormal mitoses in all the cells, whereas, in the second patient, only minor abnormalities were found. A review of previously published cases of breast cancer with acute leukemia was done. Our findings suggest that the association of leukemia with breast cancer may result from therapy is some cases (secondary induced acute leukemias) while in others it may occur spontaneously.