Overall,patients with congenital cyanotic heart disease and secondary polycythemia are believed to be susceptible to vascular thrombosis[Adatia I,et al,1993], particularly following Modified Blalock-Taussig shunt operation, when platelet dysfunction may occur as common haemostatic disorder [Klinge J,et al,1995]. Blood platelets in patients with cyanotic congenital heart diseases were occasionally characterized by significantly enhanced expression of P -selectin in resting circulating platelets, as well as their augmented activation in response to stimulating agents [Rinder, et al, 1994]. Subjects and methods : This study included 47 patients with congenital cyanotic heart disease[preoperative[groupI],postoperative[group II]] and 12 age and sex matched controls.All patients were subjected to; Proper history taking ,thorough clinical assessment,chest X- ray,echocardiography,pulse oxymetry and laboratory investigation [CBC,assessment of plasma level of P- selectin and typing of genetic polymorphisms]. Results: Our study showed a statistically significant higher level of P-selectin in patients with congenital cyanotic heart disease [hyper reactive platelets] than levels observed in healthy controls [patients [8] and control [5], p<0.05]. There were a statistically significant higher levels of P-selectin in young patients below 3 years, in group I and group II than that of similar age group in control [group I [8.8],group II [12.9] and controls[4.7] p<0.05]. Statistically significant higher levels of P-selectin were found in homozygous A2A2 patients than A2A2 controls [patients [10.3] and controls [5.7] p<0.05]. Statistically significant higher levels of P-selectin were found in positive phenotype patients [group I and group II] than in positive phenotype controls. There was a significant statistical negative correlation between O2 saturation and P-selectin levels of the patients [pre operative & post operative] [r=-0.5].Conclusion : P-selectin levels and hence platelet activity are higher in congenital cyanotic heart disease patients specially those with thrombotic episodes.The hypoxia and cyanosis observed in those patients lead to platelet activation and endothelial dysfunction . We recommend estimation P- selectin level in these patients to predict those at risk of thrombosis. We recommend also the use of platelet function inhibitor [e.g.salicylates] in patients with high levels of P- selectin especially before any interventional procedure.