MicroRNA (miR)-155 is upregulated in breast cancer cells and in sera of patients with breast cancer, but its clinical relevance remains uncertain. The objective of the present effort was to address the transcriptional regulation of miR-155. A bioinformatics analysis of public datasets validated upregulation of miR-155 in tumor cells of patients with breast cancer, particularly those who were at early stages and had triple-negative cancers. The expression profiling and clinical relevance of miR-155 in tumor cells and blood cells were characterized by TaqMan miR assays and, in plasma and exosomes, by nest-quantitative PCR analysis. There was a positive correlation between expression of FOXP3 and miR-155 in breast cancer cell lines and primary breast cancers. In breast cancer cells, FOXP3 induced miR-155 through transcriptional repression of BRCA1. Furthermore, in an Alabama cohort, blood and plasma samples were collected from 259 participants, including patients with breast cancer or benign breast tumors, members of breast cancer families, and matched healthy female controls. For patients with early stage or localized breast cancer, there were high levels of miR-155 in both plasma and blood cells. In cultured breast cancer cells, expression of miR-155 was induced by FOXP3 but was not significantly changed in culture medium or exosomes, suggesting that circulating miR-155 originated from blood cells. These findings reveal a transcriptional axis of FOXP3-BRCA1-miR-155 in breast cancer cells and show that plasma miR-155 may serve as a non-invasive biomarker for detection of early stage breast cancer.