To see whether the clinical manifestation of thrombotic events or hemorrhagic infarctions appears as a relevant problem when treating children with acute lymphoblastic leukemia (ALL) concerning the COALL therapy-protocol, we started an inquiry of the participating hospitals. The mentioned protocol was designed by the German Society for Pediatric Oncology and Hematology to treat ALL in childhood. All participants gave us information about the treatment period from January 1989 to December 1992. In 6 from 286 treated patients a thromboses appeared in clinical terms. None of them was connected with a lethal outcome. There was no observation of a hemorrhagic infarction. The overall thromboses frequency was 2.1%. In 1.4% patients "symptomatic" thrombosis developed close to a continuous venous catheter, which can be considered as a thrombogene risk factor. About 0.6% (2/286) of the patients developed the thrombotic events without another risk factor. They can be regarded as "idiopathic". 1/2 idiopathic thromboses led to a life threatening situation. There are two important factors that can enhance thromboses: 1) the therapy period, especially induction therapy and application of asparaginase and 2) a continuous venous catheter. The fact that asparaginase is not used during the induction therapy is a characteristic of the COALL protocol. It seems to be useful to differentiate between "idiopathic" and "symptomatic" thrombotic events, because "symptomatic" thromboses appear also in non-leukemic diseases quite frequently.