Patients with acute leukemia develop abnormalities of haemostasis, leading not only to bleeding, but also to thrombotic complications. The pathogenesis of these complications is complex and multifactorial. Because platelets and platelet derived microparticles are key players in haemostasis and thrombosis, we presumed their roles in the prediction of bleeding and thrombotic complications. Our study groups included 24 patients with acute leukemia and 16 healthy volunteers. Platelet aggregation evaluation was performed by impedance whole blood aggregometry and the ennumeration of platelet derived microparticles was done by means of flow cytometry. Eight patints developed hemorrhagic complications associated with reduced platelet aggregation response at hospital admission. Major thrombotic events occurred in 5 patients, being preceded by increased platelet aggregation in 3 cases and high level of platelet derived microparticles in 2 cases. Our findings reveal that whole blood platelet aggregometry could be a valuable tool especially in the detection of platelet hyperreactivity and in the prediction of thrombotic events. A high level of platelet derived microparticles could also predict thrombosis. These hypotheses need further evaluation and confirmation on larger number of patients.