Objective: To investigate the proportional changes of CD4+CD25+ regulatory T cells in peripheral blood after lymphocyte therapy in unexplained recurrent spontaneous abortion (URSA) patients. Design: Prospective cohort study. Setting: University Hospital. Patient(s): Twenty-five URSA patients. Intervention(s): Measurements of CD4+CD25+ regulatory T cells in peripheral blood before and after paternal or third-party lymphocyte immunization. Main Outcome Measure(s): The proportion of CD4+CD25bright regulatory T cells and the percentage of CD25bright cells in the CD4+ T-cell population. Result(s): The proportion of CD4+CD25bright T cells in peripheral blood from URSA patients was increased significantly after paternal or third-party lymphocyte immunization therapy, whereas the percentage of CD4+CD25dim cells were decreased significantly. The percentage of CD4+CD25bright cells in the CD4+ T-cell population was significantly increased, and the proportion of CD4+CD25bright T cells was significantly higher in successfully pregnant women than in those with pregnancy loss after lymphocyte therapy. Conclusion(s): Allogeneic lymphocyte therapy can enhance the percentage of CD4+CD25bright regulatory T cells in peripheral blood, therefore CD4+CD25+ regulatory T cells may serve as a novel biomarker for monitoring allogeneic lymphocyte therapy in URSA patients. © 2009 American Society for Reproductive Medicine.