In a previous study, we demonstrated that the proportion of activated T cells (CD69+CD3+ and HLA-DR+CD3+) is higher in the endometrium and decidua after the luteal phase and throughout early pregnancy compared with in the peripheral blood. However, there was no difference in the proportion of CD25+CD3+ lymphocytes between the endometrium and peripheral blood. In this study, we further verify that the levels of CD25 on CD4+ and CD8+ T lymphocytes are not increased in normal pregnancy, although the levels of CD69 and HLA-DR are markedly increased. We also elucidate that the amounts of all three activation molecules on local T lymphocytes are down-regulated in pregnancy compared with that during the luteal phase. Nevertheless, these decreases are significantly lessened in anembryonic pregnancies with both normal and abnormal karyotyping. However, in peripheral blood, the down-regulation of activation molecules levels in pregnancy is only demonstrated on CD4+ cells and for HLA-DR on CD8+ cells. Furthermore, dual activation marker analysis demonstrated that the expression of CD25 appears to be dissociated from CD69 and HLA-DR on the same decidual lymphocytes. Because IL-2Ralpha plays a pivotal role in the development and propagation of functional T cells, its depressed expression may result in maternal tolerance of the fetal allograft.