Although cardiopulmonary disease is associated with decreased functional capacity among adults with sickle cell disease (SCD), its impact on functional capacity in children with SCD is unknown. We evaluated 6-min walk (6MW) distance in 77 children and young adults with SCD undergoing screening for cardiopulmonary disease. Of 30 subjects who also underwent cardiopulmonary exercise testing, we found evidence for decreased exercise capacity in a significant proportion. Exercise capacity was related to baseline degree of anemia and was significantly lower in subjects with a history of recurrent acute chest syndrome. We found that 6MW distance adjusted for weight and body surface area was shorter in subjects with restrictive lung disease but that only 6MW adjusted for weight remained significantly shorter when we controlled for baseline hemoglobin. Exercise capacity was not significantly different in subjects with and without cardiopulmonary disease. We conclude that restrictive lung disease is associated with shorter 6MW distances in children and young adults with SCD, but that variables associated with decreased exercise capacity, other than anemia, remain unclear. Our study underscores the importance of further delineating the direct pathophysiologic processes that contribute to decreased exercise capacity observed among individuals with SCD and cardiopulmonary disease.