Objective: To investigate the proportions of CD4+CD25 high T cells and forkhead box p3 (FOXP3) expression in peripheral blood and decidua in patients with unexplained recurrent spontaneous abortion (URSA). Design: Prospective clinical study. Setting: University hospital. Patient(s): 125 URSA patients, 35 normal early pregnant women, and 28 normal nonpregnant women. Intervention(s): Measurements of CD4+CD25 high T cells and FOXP3 expression in peripheral blood and decidua. Main Outcome Measure(s): The proportions of CD4+CD25high T cells and FOXP3 expression. Result(s): In peripheral blood, statistically significantly higher proportions of CD4+CD25high T cells and FOXP3 expression were observed in normal early pregnant women compared with normal nonpregnant women and URSA patients; a statistically significantly lower proportion of CD4+CD25high T cells was observed in nonpregnant URSA patients compared with URSA patients who had early miscarriages and normal nonpregnant women. In the decidua, statistically significantly lower proportions of CD4+CD25high T cells and FOXP3 expression were found in URSA patients with early miscarriages compared with normal early pregnant women. Conclusion(s): The CD4+CD25high T cells may play an important role in maintaining a normal pregnancy. The reduction in CD4+CD25high T cells may involve in the pathogenesis of URSA, and is correlated with lower FOXP3 expression. Copyright © 2010 American Society for Reproductive Medicine, Published by Elsevier Inc.