Persistence of human fetal hemoglobin (HbF, alpha(2)gamma(2)) in adults lessens the severity of sickle cell disease (SCD) and the beta-thalassemias. Here, we show that the repressor BCL11A is required in vivo for silencing of gamma-globin expression in adult animals, yet dispensable for red cell production. BCL11A serves as a barrier to HbF reactivation by known HbF inducing agents. In a proof-of-principle test of BCL11A as a potential therapeutic target, we demonstrate that inactivation of BCL11A in SCD transgenic mice corrects the hematologic and pathologic defects associated with SCD through high-level pancellular HbF induction. Thus, interference with HbF silencing by manipulation of a single target protein is sufficient to reverse SCD.