The purpose of this study is to investigate the hemostatic side effects of HDMTX. Between 2001 and 2002, 20 children with acute lymphoblastic leukemia at the Dr. Sami Ulus Children's Hospital, Department of Pediatric Hematology and Oncology, treated according to the St. Jude ALL XIII protocol were eligible to this study. Methotrexate at a dose of 2 g/m2 was infused over 24 hours. Coagulation screening studies included prothrombin time (PT), APTT, fibrinogen, fibrin degradation product (D-Dimer), factor II, factor V, factor VII, factor VIII, factor IX, factor X, PC, PS, AT-III determinations before HDMTX therapy (PreT), 1 day after (Post1D), and 1 week after (PostT1W) the end of the HDMTX infusion. We found that PT and APTT were prolonged, PC, PS, and AT-III levels were decreased with a slight increase in D-Dimer 1 day after the administration of HDMTX and all of them returned to the normal levels by 7 days. In addition we found that FVII, FIX, FX were significantly decreased 1 day after therapy and normalised by 7 days.