<qa>  What is the most serious complications of sickle cell disease?Cerebral vasculopathy and stroke are among the most serious complications of sickle cell disease. Not every patient will develop central nervous system disease but most children have some evidence of cognitive impairment and about 10 will have overt stroke if this is not forestalled by transcranial Doppler TCD screening, identification of individuals at greatest risk by estimating maximal cerebral blood flow and commencing prophylactic transfusion. The basis of these recommendations is studies in children over age 2-years where stroke is most prevalent. Transfusion is not without its own serious and considerable risks so that other measures for delaying or preventing cerebral vasculopathy would be valuable. Hydroxyurea HU can reduce both morbidity and mortality of sickle cell anemia in adults; most likely the same pertains in children. Unknown are the effects of HU in infants and the youngest children and whether its successful use can reduce the risks of sickle cerebrovascular disease. If HU is associated with reduced TCD velocity, and if it is safe in infants, the medical implications would be high. 