L-asparaginase is commonly used in the chemotherapy regimens for acute lymphoblastic leukaemia. Its use is associated with thrombotic complications in 1 to 14 % of the cases. The pathogenesis of this complication is still unclear. However, the decrease of antithrombin seems to play an important role. We report a case of a 17-year old man with a acute lymphoblastic leukaemia, who developed a cerebral sinovenous thrombosis due to an acquired deficiency of antithrombin and protein C and S following L-asparaginase chemotherapy. We discuss the use of prophylactic supplements of antithrombin and the value of screening of thrombophilia based on the recent medical literature.