 The investigators would like to study the endothelial function in sickle cell patients without pulmonary hypertension in an in vivo method during a steady state condition before and after sildenafil treatment for 1 month, and to study the effects of this nitric oxide donor by measuring the Flow Mediated Dilatation, by measuring endothelial progenitor stem cells colonies, and by measuring the effect of therapy on markers of inflammation cell adhesion molecules and cytokines. 