Posttransplant cyclophosphamide (PTCy) is an effective prophylaxis for graft-versus-host disease (GVHD) due to its suppression of donor-derived alloreactive T cells in allogeneic hematopoietic stem cell transplantation (HSCT). The graft-versus-leukemia (GVL) effect is an antileukemia effect induced by donor-derived alloreactive T cells, similar to GVHD, whereas no studies have demonstrated the association between the dynamics of donor-derived alloreactive T cells and impairment of the GVL effect after HSCT with PTCy. We herein evaluated the dynamics of donor-derived T cells expressing a functional marker for alloreactivity, programmed cell death-1 (PD-1), in a murine HSCT model with PTCy. We showed that PTCy was associated with the development of leukemia cells and the decreased survival probability in an HSCT model with leukemia cells, whereas PTCy could ameliorate GVHD and increased the survival probability in the HSCT model without leukemia cells. We revealed that the percentages of PD-1 expressing donor-derived CD8+/CD4+ alloreactive T cells, with the exception of CD44+ memory T cells, in the recipient spleen were suppressed with PTCy, and that donor T-cell chimerism levels were decreased early after HSCT with PTCy. Our results suggest that PTCy correlated with the impairment of the GVL effect and the amelioration of GVHD through the suppression of PD-1 expressing donor-derived CD8+/CD4+ alloreactive T cells after HSCT.