The research presented hen investigated platelet activation in cyanotic and acyanotic congenital heart diseases (CHD). Children with cyanotic CHD are prone to both thrombosis and hemorrhage. However, patients with acyanotic CHD may also have a mild bleeding disorder. The platelet activation in CHD was investigated in support of a hypothesis that platelet activation may play a role in the hemostatic abnormalities reported in these patients. Platelet activation was determined by using how cytometry with anti-CD62 monoclonal antibody (mAb), which has been shown to be a specific marker of platelet activation. Thirteen children with cyanotic CHD, 33 children with acyanotic CHD and 17 healthy children serving as controls were studied. Platelet activation was significantly higher in the cyanotic group and also in the acyanotic group compared with the healthy children (P = 0.0000 and P = 0.019, respectively). In the cyanotic group, platelet activation showed a direct correlation with arterial O-2 saturation (Sao(2)) (P = 0.014). There was no correlation between platelet activation and erythrocyte related parameters in either group. Platelet activation occurs in CHD, particularly in patients with cyanotic CHD (even in patients with no evidence of clinical thrombosis) and it may play a role in the pathogenesis of thrombotic disorders seen in these patients.