Venous thromboses (VT) in children with ALL who were treated according to the COALL-89 protocol were reported to occur with a frequency of 2.1% (6/286). 4/6 of the reported VT were catheter related. However, in other cohorts of ALL patients treated according to American protocols the incidence of severe thromboses was 2-11%. Most of the VT were not catheter related, but were atypical thromboses like sinus venous thromboses. In these patients hereditary thrombophilia risk factors seemed to play a major role. In a 6 year period including the COALL protocols -92 and -97 only 10/684 (1.5%) children presented with symptomatic VT, and 7/10 thromboses were catheter related. Every thrombotic event could be successfully treated either by heparin administration or fibrinolytic agents. 2/10 VT were secondary due to a septic event. 5 out of 8 primary VT occurred after asparaginase/dexamethasone application during the reinduction therapy. In conclusion, symptomatic thrombotic events are very rare in the COALL studies. Important risk factors for development of VT appeared to be central lines, asparaginase application and infectious/septic complications. However, the role of genetic risk factors of thrombosis in these patients has still to be determined.