Cytomegalovirus (CMV)-specific immune reconstitution early after stem cell transplantation (SCT) was evaluated prospectively by detecting CD8+ T-cells, which recognize the peptide QYDPVAALF in the context of HLA-A*2402. Fifteen allogeneic SCT recipients were included in the study. All recipients and donors were seropositive for CMV and had the HLA-A*2402 allele. CMV-specific T-cells were detected as early as 1 month after transplantation, and their numbers increased to peak levels 2 to 5 months after transplantation. The numbers of CMV-specific T-cells in patients who developed grade II to IV acute graft-versus-host disease (GVHD) and received corticosteroids for acute GVHD were low in the early period after allogeneic SCT. There was a trend toward earlier reconstitution of CMV-specific CD8+ T-cells in allogeneic peripheral blood SCT (PBSCT) patients than in allogeneic bone marrow transplantation patients. The contribution of T-cells in the graft to the recovery of CMV-specific immune responses was also suggested by the finding that the reconstitution of CMV-specific CD8+ T-cells was delayed in CD34-selected autologous PBSCT compared with unpurged autologous PBSCT. The reconstitution of CMV-specific CD8+ T-cells was delayed in patients with CMV disease or recurrent CMV reactivation. These observations suggest that the detection of CMV-specific T-cells with an HLA-peptide tetramer is useful to assess immune reconstitution against CMV and to identify patients at risk for CMV disease or recurrent CMV reactivation after SCT.