Mechanisms of hypercoagulability in cyanotic congenital heart disease (CCHD) are not fully elucidated, but endothelial dysfunction, hemostatic abnormalities, hyperviscosity, and iron deficiency are proposed underlying factors. We aimed to evaluate the risk factors for thrombosis in children with CCHD. Forty children with CCHD and 47 matched healthy controls were enrolled. Patients were divided into: Group I; 14 (35%) with history of thrombosis and Group II; 26 (65%) without history of

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... . Laboratory assays included complete blood count, serum iron profile and serum thrombomodulin level. CCHD patients had higher hemoglobin and hematocrit, lower mean corpuscular volume, mean corpuscular hemoglobin, platelets, iron and thrombomodulin than the control group. Group I showed higher hematocrit and lower platelets than Group II. No significant difference in iron profile and thrombomodulin was found between the patient groups. Thrombomodulin was positively correlated to oxygen saturation in all, Group I and Group II patients. CCHD is associated with a hypercoagulable state and a multifactorial risk for thrombosis. Decreased thrombomodulin is a reliable marker for endothelial dysfunction, however not a good marker for predicting thrombosis in children with CCHD. Infections and dehydration, increased hematocrit, and platelet activation are speculated to substantially participate in the prothrombotic state encountered in CCHD.