In 26 acute promyelocytic leukaemia (APL) patients treated with all-trans retinoic acid (ATRA), 23 percent had platelet counts between 459 and 800 X109 /l during treatment. These values, observed between days 28 and 45 of ATRA treatment, were transient and asymptomatic. We report two APL cases with platelet counts greater than 1000 X 109 /l during ATRA therapy who were treated with recombinant interferon alpha. In both cases ATRA doses were not modified, no complications secondary to thrombocytosis were seen, and they subsequently achieved complete remission. It is suggested that IL-6 may play an important role in the pathogenesis of the thrombocytosis induced by ATRA. To our knowledge, this is the first report of thrombocytosis occurring during ATRA treatment.