Secondary erythrocytosis is common in patients with cyanosis secondary to congenital heart disease (CHD) and/or pulmonary hypertension (PH). This compensatory mechanism aims at increasing oxygen delivery to the tissues, but it requires adequate iron stores. Optimal methods of iron supplementation in this setting remain controversial, with fears of excessive erythropoiesis and hyperviscosity symptoms. We describe our experience using intravenous ferrous carboxymaltose.