Objective laboratory tools are needed to monitor developing organ damage in sickle cell disease (SCD). Circulating endothelial cells (CECs) are indicative of vascular injury. We determined whether elevated CEC can be detected in asymptomatic SCD with the CellSearch system and whether the CEC count is related to clinical and blood-based biomarkers of disease severity. Fifteen consecutive clinically asymptomatic HbSS patients and 15 matched HbAA controls were analyzed for CEC counts, laboratory parameters of disease severity (Hb, leukocyte counts, HbF%), plasma levels of markers for endothelial activation (sVCAM-1, VWF:Ag) and of endogenous inhibitors of nitric oxide synthase (asymmetrical dimethylarginine [ADMA]). CEC counts were significantly higher in patients (12 cells/mL, IQR 8-29) as compared to controls (4 cells/mL, 3-10) (P=0.007). CEC counts were significantly higher in patients with pulmonary hypertension (PHT) (P=0.015), and increased with increasing number of affected organs (0-4 involved organs, P=0.002). No significant correlations between CEC and any other laboratory parameter were detected. In conclusion, CECs could prove to be an important new tool for assessing developing vasculopathy and organ damage in SCD.