Human Vα24+ natural killer T (NKT) cells have an invariant T-cell receptor-α chain and are activated in a CD1d-restricted manner. Vα24+ NKT cells are thought to regulate immune responses and to play important roles in the induction of allograft tolerance. In this report, we analyzed the recovery of Vα24+ NKT cells after hematopoietic stem cell transplantation and its correlation with graft-versus-host disease (GVHD). Patients who received a dose-reduced conditioning regimen, antithymocyte globulin-or CAMPATH-1H-containing conditioning regimen were excluded. NKT cells were reconstituted within 1 month after transplantation in peripheral blood stem cell transplantation recipients, while their numbers remained low for more than 1 year in bone marrow transplantation (BMT) recipients. The number of Vα24+ NKT cells in BMT recipients with acute GVHD was lower than that in patients without acute GVHD, and both the CD4+ and CD4− Vα24+ NKT subsets were significantly reduced. With regard to chronic GVHD, BMT recipients with extensive GVHD had significantly fewer Vα24+ NKT cells than other patients. Furthermore, the number of CD4+ Vα24+ NKT cells was also significantly reduced in patients with chronic extensive GVHD. Our results raise the possibility that the number of Vα24+ NKT cells could be related to the development of GVHD.