Naive T cells after allogeneic hematopoietic cell transplantation are thought to originate from the engrafted hematopoietic cells. In this report, we show that there is a correlation between the number of naive CD4 T cells infused with peripheral blood stem cell grafts and the absolute number of peripheral naive CD4 T cells on day 30 (R = 0.65; P < .001), day 80 (R = 0.63; P < .001), and day 180 (R = 0.66; P < .001) after transplantation. These results suggest that in the first 6 months after transplantation, most naive CD4 T cells are derived from the naive T cells infused with the graft. (C) 2003 American Society for Blood and Marrow Transplantation.