For relapsed acute promyelocytic leukemia after all-trans retinoic acid (ATRA) and arsenic treatment, remission can be achieved by chemotherapy with ATRA and/or arsenic and addition of mylotarg. Autologous hematopoietic cell transplantation using a polymerase chain reaction (PCR) negative graft is important treatment option to obtain sustainable remission. This study is to test the efficacy and the safety of conditioning regimen with idarubicin and busulfan for relapsed Acute Promyelocytic Leukemia (APL).