Erythrocytosis is an adaptive response to improve oxygen transport in cyanotic congenital heart disease (CCHD). However, at highly increased hematocrit levels patients may experience hyperviscosity symptoms. Iron deficiency in CCHD patients is often overlooked due to elevated hemoglobin concentrations. A 29-year-old male with CCHD was readmitted to our outpatient clinic. Red blood cells (11.65*10(12)/L), hemoglobin (25.7 g/dL), and hematocrit (80%) were extremely elevated. Measurements of iron supply showed a constellation typical for iron deficiency with low ferritin (13.2 microg/L), and high sTfR (20 mg/L). We present a case of extremely high red blood cell counts with concomitant iron deficiency. For appropriate management and to avoid misinterpretation of the iron status, ferritin and sTfR should always accomplish laboratory examination of CCHD patients.