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Complications of chronic hypoxia, including erythrocytosis, hyperviscosity, abnormalities of hemostasis, cerebral abscesses, stroke, and endocarditis, are among the most common consequences of cyanotic heart disease in adults. The compensatory erythrocytosis of cyanotic heart disease can become pathologic by causing an increase in blood viscosity, thereby decreasing perfusion and resulting in decreased total oxygen delivery and increased risk of venoocclusive/hyperviscosity syndrome. Treatment of hyperviscosity secondary to erythrocytosis in cyanotic heart disease is controversial. Data is limited but suggest that phlebotomy has the potential to increase exercise capacity, reduce the symptoms of hyperviscosity, and reduce the potential risk of vasoocclusive disease in selected patients with polycythemia secondary to cyanotic heart disease. Unfortunately, repeated phlebotomy can quickly lead to iron deficiency, resulting in microcytic erythrocytes that induce higher viscosity than normocytic erythrocytes, which may increase the risk for venoocclusive events. There are limited data on the use of hydroxyurea to suppress erythrocytosis in this patient population. The authors conclude that until newer approaches to decreasing hematocrit without inducing iron deficiency are shown to be safe and efficacious, phlebotomy should only be used for the acute resolution of hyperviscosity symptoms. In addition, the use of hydroxyurea should be limited to patients with recurrent symptoms.