oPrevious (6 months prior) central nervous system event lasting longer than 24 hours, plus objective imaging evidence of CNS vasculopathy, with or without residual neurologic findings oFrequent (3 per year for 2 years) painful vaso-occlusive episodes (defined as episode lasting 4 hours and requiring hospitalization or outpatient treatment with parenteral narcotics) oRecurrent (3 in lifetime) acute chest syndrome events which have necessitated exchange transfusion or chronic transfusion therapy. Must have failed a good-faith trial of hydroxyurea (failure defined as a reduction of less than 50% in the incidence of vaso-occlusive events over a period of at least 18 months) or have demonstrated an inability to take the drug due to side effects.