Regulatory T cells (T_regs) that constitutively express FOXP3 are instrumental to the maintenance of tolerance and may suppress graft-versus-host disease (GVHD) in humans. To determine whether regulatory T cells in allogeneic stem cell transplants (SCTs) ameliorate GVHD after transplantation, we quantitated the coexpression of FOXP3 on CD4⁺ T cells in 32 donor SCTs infused into HLA-matched siblings and examined GVHD incidence in respective recipients. High CD4⁺FOXP3⁺ T-cell count in the donor was associated with a reduced risk of GVHD. We monitored T_regs during immune reconstitution in 21 patients with leukemia undergoing a T-cell–depleted allogeneic SCT. Early after SCT, there was a significant expansion in the CD4⁺FOXP3⁺ T-cell compartment. A low CD4⁺FOXP3⁺ T-cell count early after SCT (day 30) was associated with an increased risk of GVHD, and the ratio of CD4⁺FOXP3⁺ T cells to CD4⁺CD25⁺FOXP3^– T cells was significantly reduced in patients with GVHD, suggesting diminished control of effector T cells. Our findings suggest that graft T_reg content may predict for risk of GVHD after SCT. Determining the T_reg levels in the donor and manipulating T_regs early after transplantation may provide a new approach to controlling GVHD.