The clinical efficacy of oral hydroxyurea (HU) in adults and children with sickle cell anemia (SCA) cannot solely be explained by its ability to enhance fetal hemoglobin (HbF) expression. Since increased adherence of sickle red blood cells to vascular endothelium is a possible contributing factor to vaso-occlusive crisis (VOC), we explored the effect of HU on human endothelial cell (EC) lines (TrHBMEC and EA-hy 926). We demonstrated that HU, in a dose-dependent and reversible manner, significantly decreased (up to three-fold) the release of endothelin-1 (ET-1), a vasoconstrictor peptide through downregulation (up to three-fold) of ET-1 gene expression. This finding is of therapeutic relevance as SCA patients exhibit elevated serum levels of ET-1 during episodes of VOC and levels correlate with disease severity. Unexpectedly, HU upregulated (up to three-fold) the expression of membrane-bound intercellular cell adhesion molecule 1 (mbICAM-1) and its soluble form (sICAM-1) with a parallel increase in ICAM-1 mRNA expression. Although ICAM-1 does not appear to be involved in the sickle cell adhesion to vascular endothelium, it may exacerbate vaso-occlusion by promoting leukocyte adhesion. The HU-induced increase in mbICAM-1 may appear inconsistent with the clinical benefits confered by HU. However, both the increase in sICAM-1- and HU-induced leukocyte reduction in patients, may counteract the potentially detrimental effect of elevated mbICAM-1 expression. Also HU reduces the expression of vascular cell adhesion molecule (VCAM-1) on EC. Since HU reduces the very late antigen 4-positive reticulocytes in SCA patients, a ligand for VCAM-1, HU-induced downregulation of VCAM-1 on EC will very likely decrease the reticulocyte-endothelium adhesion. Thus, HU, apart from inducing HbF expression in the red cell, also affects the expression profile of EC compartment.