We report a patient with therapy-related acute promyelocytic leukemia (APL) that may have been caused by regional radiation or hormonal therapy after surgery. A 36-year-old Japanese woman developed right breast cancer and underwent breast-conserving surgery and regional radiation to the right breast without adjuvant systemic therapy because she wished to preserve her fertility. Two years later, she developed multiple bone metastases of breast cancer and received hormonal therapy. During the second line hormonal therapy, she developed APL and received induction and consolidation chemotherapy with all-trans retinoic acid (ATRA) and a combination of anthracycline and cytarabine. After she achieved a complete remission (CR) of the APL, her bone metastases of breast cancer progressed. She received weekly paclitaxel treatments and her bone marrow function recovered. However, 9 months later, her APL relapsed; she achieved a second CR after undergoing ATRA therapy again. This patient is thought to be a rare case of secondary leukemia, since the leukemia might have been caused by hormonal therapy and regional radiation without chemotherapy.