Modern molecular technology makes it possible to detect minimal residual acute promyelocytic leukaemia (APL) when the marrow remains in complete remission (CR). Thus, an important question, and the first to be discussed, is whether patients should be treated at molecular relapse, rather than only at haematological relapse. The second question to be addressed is when the various options for treatment of relapsed APL-i.e. all-trans retinoic acid (ATRA) and anthracyclines, arsenic trioxide (ATO), gemtuzumab ozogamycin (GO, 'mylotarg'), and allogeneic or autologous stem cell transplant-should be used. We suggest that patients with relapsed APL have different prognoses and that, accordingly, they should not all be treated identically. We note the significance of extramedullary relapse. We conclude with a summary of treatment recommendations and a discussion of new therapies that might be useful in the future.