Hyperleukocytic leukemias are a small proportion of leukemias that have white blood cell count > 100 x 10(9)/l, most of them are leukemic blast cells. These leukemias have a grave prognosis because they can develop a leukostasis syndrome which describes: the acute onset of pulmonary failure and, often, neurologic deficits and disseminated intravascular coagulation (DIC). The leukostasis is produced by the mechanical obstruction of vascular bed by blast cells, which can be induced by the spontaneous tumor lysis or as a side effect of cytotoxic drugs. So, hyperleukocytic leukemias require early and vigorous measures to decrease the white blood cell count, using leukapheresis and/or chemotherapy, before pulmonary failure exists. Then, it is possible to reverse the lesions. We report two cases of acute myeloblastic leukemia with a white blood count > 100 x 10(9)/l, that developed a respiratory distress syndrome and died. The postmortem examination has been done in one of the cases.