Therapy-related acute lymphoblastic leukemia (t-ALL) with t(9;22)(q34;q11.2) is rarely reported as a secondary malignant neoplasm. To our knowledge, only 10 Ph+ t-ALL cases have been reported in the literature. However, 8/10 cases were not reported individually but constituted parts of larger studies reporting cytogenetic abnormalities in secondary leukemia. Snyder et al. first reported Ph+ t-ALL in two patients who had been treated successfully for Ewing sarcoma. We present a novel case of t-ALL with t(9;22) in a patient with primary breast cancer. The interval between diagnosis of breast cancer and the appearance of ALL was 4 years. The patient was treated with partial mastectomy and axillary lymph node dissection followed by six cycles of CMF (cyclophosphamide, methotrexate, 5-fluorouracil) chemotherapy and radiation therapy. Bone metastases were found 3 years after surgery, and she was treated with epirubicin and paclitaxel. This case has the following unique features: BCR/ABL gene rearrangement in t-ALL, and two types of malignant cells (leukemic lymphoblasts and metastatic breast cancer cells) coexisted in the bone marrow.