Changes in the coagulation system due to steroids and asparaginase during treatment for acute lymphoblastic leukemia (ALL) are well known side effects and may cause bleeding or thrombosis. We report the case history of a 7-year old girl who developed thrombosis of the sinus sagittalis superior during ALL-treatment. Diagnosis was made by computed tomography and magnetic resonance imaging after the child became symptomatic with seizure. Until this event the girl had been treated already for two weeks with prednisone and E. coli-asparaginase (4 infusions). This medication caused distinct hypofibrinogenemia (Fibrinogen 53 mg/dl), prothrombin time expressed as percent of normal values of 58% was also pathological, activated partial thromboplastin time of 35 sec, antithrombin III 120% and thrombocyte count 178 G/l were in normal range. We were not successful in the attempt to adjust the imbalance in the coagulation system by transfusion of fresh frozen plasma (FFP)--seizure happened during FFP-infusion, fibrinogen blood level could be elevated only slightly. Our patient stayed consequently asymptomatic, the clinical recovery was confirmed radiologically.