The characteristic of acute promyelocytic leukaemia (APL) is a reciprocal translocation between chromosomes 15 and 17 resulting in a chimeric PML and retinoic acid receptor alpha (RARA) oncogene. The fusion protein (PML/RARA) formed as result is thought to block the differentiation of the bone marrow cells arrested at the promyelocytic stage. It has been documented by both in vivo and in vitro studies that a large number of APL cells undergo granulocytic maturation after all-trans retinoic acid (ATRA) therapy. The case report presented here is a rare case of PML/RARA positive APL patient exhibiting extensive monocytic differentiation after ATRA therapy as documented by morphology and FISH studies.