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Background. Forkhead Box Protein 3 (FOXP3) is a marker for immunosuppressive CD4 + CD25+ regulatory T cells (Tregs). We investigated whether there were significant numbers of FOXP3-positive Tregs in triple-negative breast cancer (TNBC) using immunohistochemistry, and whether the presence of FOXP3-positive Tregs was associated with other prognostic factors, such as stage or histologic grade. We investigated the number of tumor-infiltrating FOXP3-positive Tregs in formalin-fixed TNBC specimens obtained from patients who received palliative treatment between 1999 and 2007. Material and methods. Immunohistochemistry was used to assess the number of CD4 + , CD25+, and FOXP3+ Tregs in tumor tissue and normal breast tissue from 86 TNBC patients. Univariate and multivariate analyses evaluated outcomes according to the number of FOXP3-positive Tregs. Results. Of the 86 tumor specimens, 22 (25.6%) expressed more than 15 FOXP3-positive Tregs per 10 high power fields in the peritumoral area. On multivariate analysis, staining showing > 15 FOXP3-positive Tregs was an independent prognostic factor for overall survival and progression free survival with hazard ratios of 2.4 (95% CI 1.0-5.6; p = 0.049) and 2.0 (95% CI 1.1-3.6; p = 0.032), respectively. In TNBC, FOXP3-positive Tregs had stronger prognostic significance than did FOXP3-negative Tregs. The finding of improved survival associated with highly infiltrating FOXP3-positive Tregs in TNBC contrasted with several other types of solid cancer. Conclusion. TNBC may be differently driven by FOXP3 via an immune mechanism. The inclusion of FOXP3+ Tregs may help to improve prognostication for TNBC. [ABSTRACT FROM AUTHOR]