To determine whether the leukemia-associated Wilms tumor antigen (WT1) contributes to a graft-versus-leukemia (GVL) effect after allogeneic stem-cell transplantation (SCT) for acute lymphoblastic leukemia (ALL), we studied CD8(+) T-cell responses to WT1 in 10 human lymphocyte antigen (HLA)-A*0201-positive ALL patients during the early phase of immune recovery after SCT (days 30-120). Seven of 10 patients had detectable WT1 expression in their peripheral blood (PB) before SCT by quantitative reverse-transcription polymerase chain reaction. Using WT1/HLA-A*0201 tetramers and intracellular interferon-gamma (IFN-gamma) staining, WT1(+) CD8(+) T-cell responses after SCT were found only in patients with detectable WT1 expression before SCT (5 of 7 vs. 0 of 3; P < .05). To monitor the kinetics of WT1(+) CD8(+) T-cell responses and disease regression after SCT, absolute WT1(+) CD8(+) T-cell numbers and WT1 expression were studied for each time point. The emergence of WT1(+) CD8(+) T cells was associated with a decrease in WT1 expression, suggesting a WT1-driven GVL effect. Loss of WT1(+) CD8(+) T-cell responses was associated with reappearance of WT1 transcripts, consistent with a molecular relapse (P < .001). WT1(+) CD8(+) T cells had a predominantly effector-memory phenotype (CD45RO(+) CD27(-)CD57(+)) and produced IFN-gamma. Our results support the immunogenicity of WT1 after SCT for ALL and highlight the potential for WT1 vaccines to boost GVL after SCT for ALL.