Despite improvements in therapeutic options, human T cell lymphotropic virus type 1 (HTLV-1)-related adult T cell leukaemia/lymphoma (ATLL) has a dismal prognosis. The present report concerns the case of a multirelapsing ATLL that reached a complete remission following the treatment of a secondary acute promyelocytic leukaemia with cytarabine, anthracyclin, all-transretinoic acid and arsenic trioxide. This unexpected result with a multitreated/chemorefractory disease led us to reconsider the potential therapeutic benefits of arsenic trioxide, which has demonstrated efficacy against ATLL cells.