Bleeding and septicemia are the most common causes of death in patients with acute leukemia. Additionally to thrombopenia, which is present in most patients, in part of the patients severe coagulation abnormalities are observed. The prevalence of severe coagulation abnormalities in patients with acute myeloid leukemia including all FAB types is around 12%, in patients with promyelocytic leukemia the prevalence is more than 50%. The etiology of this coagulation abnormality remains unclear. There are 3 mechanisms currently under discussion: activation of coagulation, activation of fibrinolysis and an increased activity of specific leucocyte proteases. In particular, in promyelocytic leukemia the risk of cerebral bleeding is high (10 to 15% of early deaths). The most important therapeutic consequence is an intensified thrombocyte replacement regimen. Additional therapeutic efforts, like administration of heparin or antifibrinolytic drugs, have failed to improve the prognosis of patients with severe coagulation abnormalities.