2. history of exercise-induced syncope or arrhythmias. Subjects will wait at least 2 weeks following any vaso-occlusive pain episode and 12 weeks following any disease-related complication requiring transfusion support. Individuals on hydroxyurea will be eligible. A total of 30 controls without SCA or sickle cell trait will be matched for age, sex and race and recruited from the siblings, friends or relatives of subjects enrolled on this study