Common complications of leukemia are bleeding and thrombosis, in which the number and activity of platelets undoubtedly play an important role. Probably related with their apparent structural simplicity compared with other hematological cells, the interest in platelets in malignant hematological disorders has been mainly restricted to the determination of the number of circulating platelets. However, different studies have demonstrated that numbers of platelets between 6 and 80 × 10(9) platelets/l are a poor indicator of the risk of bleeding, as this number does not give any information on the functional activity of these platelets.