Sickle cell disease (SCD) patients had higher cerebral arteries velocities (by transcranial Doppler) , higher basal blood flow of skin microcirculation , prolonged time to peak , lower percent changes after thermal and reactive hyperemia tests and higher vasomotion ( by laser Doppler flowmetry) than normal controls and B-thalassemia patients. During follow up, SCD patients on hydroxyurea therapy showed improvement of cerebral arteries velocities, basal flux, time to peak, vasodilatory reserves and vasomotion than those on conservative treatment.