Objectives: This study sought to determine the relationship between blood viscosity and iron deficiency and their impact on symptoms and exercise function in adults with cyanotic congenital heart disease. Background: Iron deficiency is believed to raise whole blood viscosity in cyanotic congenital heart disease, although available data are inconsistent. Methods: Thirty-nine cyanotic adults were prospectively assessed for iron deficiency(transferrin saturation ≤5%), hyperviscosity symptoms, and exercise capacity. Same-day measurement of whole blood viscosity and hematocrit(Hct) adjusted viscosity(cells resuspended in autologous plasma to Hct of 45%) was performed at shear rates ranging from 0.277 s-1 to 128.5 s-1. Results: Viscosity did not differ between patients with iron deficiency(n=14) and those without(n=25). Whole blood viscosity correlated with Hct(r=0.63, p< 0.001 at low shear and r=0.84, p< 0.001 at high shear) but not with red blood cell size or iron indices. Hyperviscosity symptoms were independent of iron indices but directly correlated with increased Hct-adjusted viscosity(r=0.41, p=0.01). Exercise capacity did not differ in iron-deficient patients. However, peak oxygen consumption was higher in those with Hct ≥65%(12.6±3.4 ml/kg/m2 vs. 9.8±2.6 ml/kg/m2,mean±SD, p=0.036) despite higher whole blood viscosity in these same individuals(p< 0.01 for all shear rates). Conclusions: Iron deficiency is common in cyanotic adults but does not alter viscosity. Hyperviscosity symptoms are associated with a higher Hct-adjusted viscosity independent of cell size or iron stores. Higher Hct is associated with better exercise capacity. Further work to understand the origin of hyperviscosity symptoms is warranted. Objectives: This study sought to determine the relationship between blood viscosity and iron deficiency and their impact on symptoms and exercise function in adults with cyanotic congenital heart disease. Although: Iron deficiency is believed to raise whole blood viscosity in cyanotic congenital heart disease, although available Methods: Thirty-nine cyanotic adults were prospectively assessed for iron deficiency (transferrin saturation ≤ 5%), hyperviscosity symptoms, and exercise capacity. Same-day measurement of whole blood viscosity and hematocrit (Hct) adjusted viscosity (cells resuspended in autologous plasma to Hct of 45%) was performed at shear rates ranging from 0.277 s-1 to 128.5 s-1. Results: Viscosity did not differ between patients with iron deficiency (n = 14) and those without (n = 25) . Whole blood viscosity correlated with Hct (r = 0.63, p <0.001 at low shear and r = 0.84, p <0.001 at high shear) but not with red blood cell size or iron indices. The results were independent of iron indices but directly correlated with increased Hct-adjusted viscosity (r = 0.41, p = 0.01). Exercise capacity did not differ in iron-deficient patients. However, peak oxygen consumption was higher in those with Hct ≥65% (12.6 ± 3.4 ml / kg / m2 vs. 9.8 ± 2.6 ml / kg / m2, mean ± SD, p = 0.036) higher higher blood viscosity in these same individuals (p <0.01 for all shear rates). Conclusions: Iron Hyperviscosity symptoms are associated with a higher Hct-adjusted viscosity independent of cell size or iron stores. Further work to understand the origin of hyperviscosity symptoms is warranted.