One of the major problems following allogeneic stem cell transplantation (allo-SCT) is the inability to reconstitute an adequate immune system for an extended period. T-cell reconstitution is also delayed for years, especially in CD4+ T cells. In addition to impaired thymic function, shortened Naive T cell survival due to altered T cell homeostasis is reported to be responsible for delayed immune reconstitution. To further investigate the mechanisms of delayed immune recovery after allo-SCT, we focused on the frequencies of effector CD4+ T cells, because according to the previous reports, progressive linear differentiation model of CD4+ T cell predicts the accumulation of terminally differentiated effector cells when transition from naïve to memory T cells and memory to effector cells are accelerated. By flowcytometric analyses we confirmed that CD27−CD4+ T cells from allo-SCT recipients uniformly express CD95, with negative expression of CCR7 and CD62L. They also produce g-interferon (IFNg) in response to the immobilized anti-CD3 and soluble anti-CD28 stimulation, which is consistent with previous reports insisting that CD27−CD4+ T cells are functionally differentiated effector T cells. Measuring the ratio of CD27−CD4+ T cells among CD4+ T cells revealed that, although healthy donors and patients received allo-SCT within a year had comparable CD27+CD4+T-cell rate (90% vs. 83%, P=0.4436), significantly decreased rate was observed in patients transplanted more than 1 year before (55% vs. 83%, P=0.0005). The ratio of CD27+CD4+ T cells kept low during the first 5 years after allo-SCT, and then it slowly begun to increase. In addition, in patients who received stem cell grafts more than 1 year before, the ratio of CD27+CD4+ T cells were significantly higher in patients transplanted from HLA-matched siblings than in those received unrelated grafts (69% vs. 42%, P=0.0002). Other factors, such as stem cell source (BM or PBSC), patient age, and the presence of chronic GVHD did not influence the ratio of CD27+CD4+ T cells. To further investigate the characteristics of CD27−CD4+ T cells in post-transplant periods, peripheral CD4+ T cells from patients who had received allo-SCT more than 1 year before as well as healthy volunteers were sorted into CD27− and CD27+ fractions, stained with CFSE, and stimulated with immobilized anti-CD3 and soluble anti-CD28 antibodies. CD27−CD4+ T cells proliferated more vigorously at 3 days after stimulation, though after another 2-day culture, there was no difference in cell divisions between both cell groups. In addition, CD27+ cells from transplanted patients lost their expression more frequently than those from volunteers, while none of the CD27− cells stored its expression. The fact of one-way transition from CD27+ to CD27− also supported that CD27−CD4+ T cells are terminally differentiated T cells. The finding that the frequencies of CD27−CD4+ T cells begin to elevate at 1 year after allo-SCT indicates that T cells infused with allograft do not easily lose the surface expression of CD27, while T cells derived from donor’s stem cells do. Considering the fact that ratio of CD27−CD4+ T cells is much higher in recipients of unrelated grafts, and it gradually begin to decrease at 5 years after allo-SCT, the increased ratio of CD27−CD4+ T cells may reflect altered T cell homeostasis. The serial monitoring of the ratio of CD27−CD4+ T cells after allo-SCT may be useful in evaluating immune reconstitution status.