Tricuspid regurgitant (TR) jet velocity and its relationship to pulmonary hypertension has been controversial in sickle cell disease (SCD). Plasma free hemoglobin is elevated in SCD patients and acutely impairs systemic vascular reactivity. We postulated that plasma free hemoglobin would be negatively associated with both systemic and pulmonary endothelial function, assessed by flow-mediated dilation (FMD) of the brachial artery and TR jet velocity, respectively. Whole blood viscosity, plasma free hemoglobin, TR jet, and FMD were measured in chronically transfused SCD pre- and posttransfusion (N = 25), in nontransfused SCD (N = 26), and in ethnicity-matched control subjects (N = 10). We found increased TR jet velocity and decreased FMD in nontransfused SCD patients compared with the other 2 groups. TR jet velocity was inversely correlated with FMD. There was a striking nonlinear relationship between plasma free hemoglobin and both TR jet velocity and FMD. A single transfusion in the chronically transfused cohort improved FMD. In our patient sample, TR jet velocity and FMD were most strongly associated with plasma free hemoglobin and transfusion status (transfusions being protective), and thus consistent with the hypothesis that intravascular hemolysis and increased endogenous erythropoiesis damage vascular endothelia.