In Eisenmenger's syndrome a central left-to-right shunt in the heart, a congenital anomaly, leads to pulmonary hypertension which subsequently causes the shunt to be reversed. The hypoxaemia resulting from a right-to-left shunt is compensated by an increase of the haemoglobin concentration due to a rise of the haematocrit. In adult patients not operated (adequately), the symptoms are the consequence of the erythrocytaemia and an increased haemorrhagic diathesis. In the long run heart failure develops. Phlebotomy is indicated for patients with haematocrits higher than 0.65 with signs of hyperviscosity and is also advised before non-cardiac surgery to improve coagulation parameters. Phlebotomy should be performed slowly (500 ml in 30-45 min) with simultaneous volume replacement. Excessive phlebotomy causes iron deficiency and spherocytosis which increase viscosity as well as the risk of CVA. Treatment consists of iron supplementation. Anticoagulation is indicated only in case of atrial fibrillation or mechanical valves. The use of acetylsalicylacid or NSAIDs is relatively contraindicated, because of abnormal haemostasis in these patients. During treatment with ACE inhibitors and other vasodilators, hypovolaemia should be avoided, because at a lower systemic blood pressure the right-to-left shunt increases and a potentially fatal cyanosis may occur.