The incidence of complications pre- and post-hydroxyurea therapy will be also analyzed including: stroke, silent cerebral infraction, acute chest syndrome, vaso-occlusive crisis, hospitalization, pulmonary hypertension, leg ulcers, bone necrosis, and kidney injury. Safety data included adverse events as reported by the treating physician and the incidence of malignancy or death as well as pregnancy incidents and their outcomes will be also pointed out.