This study was conducted to evaluate some anticoagulants' (protein C, protein S and antithrombin) activities among epileptic children treated with VPA and to learn if thromboembolic events occur among these children or their parents. Twenty-five boys and 15 girls using VPA for at least 3 months were included in the VPA group. The control group consisted of 28 boys and 12 girls who had no infection and did not use any medication that could alter protein C activity. Complete blood counts, ALT, AST, PT, aPTT, fibrinogen, protein C, protein S and AT tests were studied in both groups. Serum VPA levels were determined in the VPA group. Protein C and protein S activities of the children in the VPA group were significantly lower than those in the control group (89.5 +/- 19.3% vs 104.9 +/- 21.7% and 44.6 +/- 16.3% vs 59.4 +/- 28.4%, respectively). Neither children using VPA, nor their parents had any thromboembolic events in medical history. Platelet counts, ALT and fibrinogen levels in the VPA group were significantly lower than those in the controls. A negative correlation was found between serum VPA level and platelet counts. There were also negative correlations between fibrinogen and serum VPA levels, and between fibrinogen level and protein S activity. The children in the VPA group had lower PT and higher aPTT levels than the children in the control group. Since other factors known to alter the anticoagulant activities and liver functions were eliminated initially, the decreases of protein C and protein S activities, thrombocyte counts, ALT, PT and fibrinogen levels and increase in aPTT level may be attributed to VPA. VPA hepatotoxicity can be the cause of decreased pro- and anticoagulant activities. (c) 2006 Elsevier B.V. All rights reserved.