Substantial evidence has demonstrated immune defects in breast cancer patients. They have decreased numbers of peripheral blood lymphocytes, but higher numbers of functionally suppressive CD4+CD25+ Treg in both peripheral blood and tumor microenvironment. Constitutive high expression of CD25 is a pivotal characteristic of natural Treg cells. This study aims at investigating if CD25 variability affects breast carcinogenesis. Two polymorphisms (rs7072793 C > T, rs3118470 C > T) in the promoter of CD25 were selected and analyzed by a multiple independent case-control study to assess the association between CD25 genotypes and breast cancer risk. Genotyping a total of 1110 patients and 1060 healthy controls in Chinese populations showed that rs7072793 CT genotype had an odd ratio of 1.49 (95% confidence interval, 1.23–1.89) for developing breast cancer compared with CC genotype, the rs7072793 TT carriers had a further increased risk of breast cancer (OR = 2.11; 95% CI = 1.66–2.87). Furthermore, our transient transfection which focused on reporter gene expression modulated by CD25 promoter demonstrated that the presence of an rs7072793 T allele led to greater transcriptional activity than the C allele. Similarly, rs13347 T carriers were shown to have larger proportion of CD4+CD25+ Tregs in the PBMCs than C carriers in the flow cytometry analysis. However, no significant differences were found in genotype frequencies at rs3118470 C > T site between cases and controls. Our findings suggest that rs7072793 C > T genetic variation in CD25 genes may be genetic modifier for developing breast cancer. © 2011 Wiley Periodicals, Inc.