Objective To investigate the changes in blood coagulation during cardiopulmonary bypass (CPB) in children of different ages undergoing open heart surgery for cyanotic congenital heart disease.Methods Sixty children with cyanotic congenital heart disease undergoing open heart surgery under CPB were divided into 3 age groups: Group A(age≤12 mort, n=25), Group B (12mon＜age≤24 mon, n= 17) and Group C (24 mon＜ age＜4 yr, n=18). Venous blood samples were taken immediately after induction of anesthesia(T1) and at 10 min after protamine administration (T2)for determination of activated coagulation time (SonACT), clot rate and platelet function (PF) using Sonoclot coagulation and platelet function analyzer-type DP2951 (Sieuco Co., USA).Results There was significant difference in SonACT, clot rate and PF at T1 among the 3 groups: the SonACT was significantly shorter in Groups B and C than in Group A, the clot rate was significantly higher in Group B than in Group C, and the PF was significantly lower in Group C than in Group A. At T2 , the SonACT was significantly prolonged in all 3 groups, the clot rate was significantly decreased in Groups A and B, and the PF was significantly decreased in Group A.Conclusion There are significant differences in blood coagulation and PF among the 3 different age groups of children with cyanotic congenital heart disease after induction of anesthesia and CPB has different effects on their blood coagulation and PF.