Background Estrogen as well as CD4(+)Foxp3(+) regulatory T cells were shown to have a protective role not only in maintaining maternal-fetal tolerance but also against autoimmune diseases. We aimed to investigate whether the pregnancy levels of estrogen are enough to induce transplant tolerance as to maintain fetal-maternal tolerance. Methods We established H-Y skin graft transplantation in C57BL/6 ovariectomized mice that reconstituted with estrogen. Subsequently, consecutive daily estrogen injection was administrated. Tregs and the cytokines in the peripheral blood were detected by flow cytometry and ELISA pre- and post-transplant. Results The results indicated that pregnancy levels of estrogen could promote Tregs in secondary lymphoid organs and peripheral blood (P <0.05) but not thymus (P >0.05). The estrogen-treated recipients accepted H-Y skin grafts for more than 35 days (median survival time (MST): (44.0+/-1.2) days) compared with estrogen-untreated mice (MST: (23.0+/-1.6) days) (P <0.05). It was also observed that estrogen up-regulated the expression of Foxp3, but did not affect CD3(+)CD8(+) effector T-cells in non-transplant mice. While in the presence of H-Y antigens, the expression of Foxp3 was more significant and CD3(+)CD8(+) effector T cells were decreased significantly (P <0.05). Meanwhile, the up-regulated IL-10 and IL-4, and down-regulated IFN-gamma could be observed (P <0.05). Conclusions Pregnancy levels of estrogen may promote the conversion of peripheral Tregs in secondary lymphoid organs, but show no effect on the natural Tregs production, differentiation and maturity in central lymphoid organs. Furthermore, pregnancy levels of estrogen could significantly prolong the survivals of H-Y skin grafts by the expansion of Tregs, suppression of CD3(+)CD8(+) effector T-cells and immune shift towards Th2 cytokines. Chin Med J 2010;123(18):2593-2599