In the present study, we examined changes in the expression of CD45RA, CD31, and CD29 on total CD4 and CD8 lymphocytes in patients who had received CD6 T cell-depleted allogeneic marrow and received no immune suppressive drugs after engraftment in order to identify defects in reconstitution of immunoregulatory T cells after allogeneic BMT. Results following allo-BMT were compared with normal controls and patients following autologous BMT. We showed that CD4+CD45RA+, CD4+CD29+ (CD29high), and CD4+CD31+ cells were markedly decreased during the first 24 months after allo- and auto-BMT. CD8+CD45RA+ cells recovered to normal levels within the first month after auto-BMT, while after allo-BMT, the CD8+CD45RA+ cells were at slightly low levels during the first month, but gradually increased to normal levels by 12 months post-BMT. CD8+CD29+ cells were increased during the first 12 months both after allo- and auto-BMT although during the first month, a decreased percentage of CD8+CD29+ cells was observed in allo-BMT patients. More important, CD4+CD29+, CD8+CD29+, and CD8+S6F1+ cells were significantly increased in patients with moderate-to-severe acute GVHD (grades II-IV) compared with those with or without mild acute GVHD (grade I), suggesting that CD4 helper-inducer (CD4+CD29high) and CD8 killer-effector (CD8+CD29highS6F1+) cells play an important role in the pathophysiology of acute GVHD.