Stroke represents a focal brain insult whereas chronic encephalopathy represents a diffuse brain disturbance involving cognition and memory. The predisposition to stroke and the potential for dementia were increasingly apparent from our studies and the work of other investigators in sickle cell disease. The ontogeny of these two processes required further study before the mechanisms could be clearly articulated; and a coordinated assessment of cerebrovascular perfusion was needed to decipher the relationship between focal perfusion deficits and occlusion of large and small cerebral vessels. 150 subjects were enrolled in these studies. Improved understanding of the mechanisms underlying these two devastating neurological complications of SCD should result in prevention or effective treatment.