Hemostatic changes in 20 children with acute lymphoblastic leukemia (ALL) who were induced with L-asparaginase (L-asp). vincristine (VCR), and prednisone (PDN) (Group A) were prospectively evaluated. These data were compared with those of a previous group of ALL patients who received L-asp as a single agent during consolidation (Group B). In Group A patients, mean plasma antithrombin activity decreased in the first 2 weeks, though not significantly. Relative to pretreatment values, mean fibrinogen concentration diminished particularly by week 3 (p < 0.001). Activated partial thromboplastin time (APTT) decreased in the last week as well as after cessation of therapy with L-asp (p < 0.05). Mean platelet count increased significantly by week 3 (p < 0.05). Thromboelastograms performed in seven patients confirmed the tendency for thrombosis evidenced by a decreased APTT. Patients in Group B (L-asp alone during consolidation) had decreased concentrations of fibrinogen, AT. and Factors IX and × after L-asp therapy. APTT was prolonged. Our data demonstrate that the tendency for thrombosis is the predominant manifestation of L-asp induced coagulopathy, when the drug is associated with VCR and PDN. Thus the risk/benefit ratio for the use of L-asp early in induction in children with low risk ALL needs to be further evaluated.