Erythrocytosis, thrombocytopenia, platelets function defects, coagulation factors deficiencies are the main haematologic disorders in patients with cyanotic congenital heart disease. They are responsible for increased bleeding tendency and contraindication of anticoagulation. Phlebotomy in this group of patients should be recommended when erythrocytosis is accompanied by clinical symptoms. Excessive phlebotomy can lead to iron deficiency and cause hyperviscosity symptoms. The iron supplementation should be closely monitored.