CD4⁺CD25⁺FOXP3⁺ regulatory T cells (Treg) successfully control graft-versus-host-disease (GVHD) in animal models. In humans, incomplete reconstitution of Treg after allogeneic hematopoietic stem cell transplantation (HSCT) has been associated with chronic GVHD (cGVHD). Recent studies have demonstrated that interleukin (IL)-2 infusions expand Treg in vivo. However, the effectiveness of this therapy depends on the number of cells capable of responding to IL-2. We examined the effect of low-dose IL-2 infusions on Treg populations after HSCT in patients who also received infusions of donor CD4⁺ lymphocytes. Utilizing FOXP3 as a Treg marker, we found that patients who received CD4+DLI concomitantly with IL-2 had greater expansion of Treg compared to patients who received IL-2 (P = .03) or CD4⁺DLI alone (P = .001). FOXP3 expression correlated with absolute CD4⁺CD25⁺ cell counts. Moreover, expanded CD4⁺CD25⁺ T cells displayed normal suppressive function and treatment with CD4⁺DLI and IL-2 was not associated with GVHD. This study suggests that administration of low-dose IL-2 combined with adoptive CD4⁺ cellular therapy may provide a mechanism to expand Treg in vivo.