The purpose of this study was to identify and validate circulating microRNAs (miRNAs) in human plasma for use as breast cancer (BC) biomarkers and to analyze their relationship to clinicopathologic features and its preliminary biological function. Genome-wide expression profiling of miRNAs in BC was investigated by microarray analysis. miR-155 was up-regulated greater than two-fold in BC compared with Normal Adjacent Tissue (NAT), whereas let-7b, miR-381, miR-10b, miR-125a-5p, miR-335, miR-205 and miR-145 were down- regulated greater than two-fold. Our hypothesis was that circulating miRNAs are also present and differentially expressed in the serum of BC patients compared to controls. Using real-time PCR (RT-PCR), we analyzed miR-205 and miR-155 in archived serum from 30 participants, 20 with breast cancer and 10 healthy people. miR-205 was down-regulated in BC patient serum while miR-155 was up-regulated. Furthermore, we analyzed the relationship between the expression levels of these two miRNAs and the clinicopathologic parameters of BC patients. High expression of miR155 was associated with clinical stage, molecular type, Ki-67 and p53 in BC patients (P<0.05). By contrast, we found no significant correlation between miR-205 and BC patient clinicopathologic parameters. Functional analysis showed that ectopic expression of miR-205 significantly inhibits cell proliferation and promotes apoptosis. miR-205 was down-regulated and miR-155 was up-regulated in BC patient serum. miR-155 was positive correlated with clinical stage and ki-67 and negatively correlated with p53 status. The purpose of this study was to identify and validate circulating microRNAs (miRNAs) in human plasma for use as breast cancer (BC) biomarkers and to analyze their relationship to clinicopathologic features and its preliminary biological function. Genome-wide expression profiling of miRNAs in BC miR-155 was up-regulated more than two-fold in BC compared with Normal Adjacent Tissue (NAT), Wnt let-7b, miR-381, miR- 10b, miR- 125a-5p, miR- Our hypothesis was that circulating miRNAs are also present and differentially expressed in the serum of BC patients compared to controls. Using real-time PCR (RT-PCR miR-205 was down-regulated in BC patient serum while miR-155 was up-regulated. Furthermore, we analyzed the relationship between the expression levels of these Two of the miRNAs and the clinicopathologic parameters of BC patients. High expression of miR155 was associated with clinical stage, molecular type, Ki-67 and p53 in BC patients (P <0.05). By contrast, we found no significant correlation between miR- BC patient clinicopathologic parameters. Functional analysis showed that ectopic expression of miR-205 significantly inhibits cell proliferation and promotes apoptosis. MiR-205 was down-regulated and miR-155 was up-regulated in BC patient serum. clinical stage and ki-67 and negatively correlated with p53 status.