ABSTRACT: INTRODUCTION: MicroRNAs (miRNAs) are a group of small non-coding RNAs involved in the regulation of gene expression. As such, they regulate a large number of cellular pathways and deregulation or altered expression of miRNAs is associated with tumourigenesis. In the current study we evaluate the feasibility and clinical utility of circulating miRNAs as biomarkers for the detection and staging of breast cancer. METHODS: miRNAs were extracted from a set of 84 tissue samples from patients with breast cancer and 8 normal tissue samples obtained after breast reductive surgery. After reverse transcription and pre-amplification, a total of 768 miRNAs were profiled using the TaqMan low-density arrays. After data normalization, unsupervised hierarchical cluster analysis (UHCA) was used to investigate global differences in miRNA expression between cancerous and normal samples. Using fold-change analysis, the most discriminating miRNAs between both tissue types were selected and their expression was analyzed on serum samples from 20 healthy volunteers and 75 patients with breast cancer, including 16 patients with untreated metastatic breast cancer. miRNAs were extracted from 200uL of serum and reverse transcribed and analyzed in duplicate using qRT-PCR. RESULTS: UHCA showed major differences in miRNA expression between tissue samples from patients with breast cancer and tissue samples from breast reductive surgery (P<0.0001). Generally, miRNA expression in cancerous samples tends to be repressed when compared to miRNA expression in healthy controls (P=0.0685). The 4 most discriminating miRNAs by fold-change (miR-215, miR-299-5p, miR-411, and miR-452) were selected for further analysis on serum samples. All miRNAs at least tended to be differentially expressed between serum samples from patients with cancer and serum samples from healthy controls (miR-215: P=0.094; miR-299-5P: P=0.019; miR-411: P=0.002 and miR-452: P=0.092). For all these miRNAs, except for miR-452, the greatest difference in expression is observed between serum samples from healthy volunteers and serum samples from untreated patients with metastatic breast cancer. CONCLUSIONS: Our study provides a basis for the establishment of miRNAs as biomarkers for the detection and eventually staging of breast cancer through blood borne testing. We identified and tested a set of putative biomarkers of breast cancer and demonstrated that altered levels of these miRNAs in serum from patients with breast cancer are particularly associated with the presence of metastatic disease.