It is well known that L-asparaginase (L-Ase) treatment may cause thrombotic events in patients with acute lymphoblastic leukemia (ALL). The mechanism of this effect is not well understood although a reduction in plasma antithrombin III (AT III) levels is observed. In our study, a group of patients treated with L-Ase received AT III concentrates as adjuvant treatment. This adjuvant treatment reduced the levels of plasma D-dimer and thrombin-antithrombin complex, which are considered as early markers of a hypercoagulability state. These preliminary data suggest that large randomized trials will have to be conducted to improve our understanding of the role of AT III concentrates in ALL therapy.