26 Background: Early detection is crucial to improve the survival rate and quality of life of breast cancer (BC) patients. Changes in DNA methylation in peripheral blood could be associated with malignancy at early stage. We aim to identify BC-associated DNA methylation signatures in peripheral blood. METHODS We identified a BC-associated differentially methylated locus by genome-wide investigation using Illumina 27K Methylation Assay. Two validation studies and replications in leucocytes and T cells were carried out using MassARRAY. The RNA expression levels were measured by real-time PCR. RESULTS The methylation level of CpG site cg27091787 in hyaluronoglucosaminidase 2 gene (HYAL2) in peripheral blood was significantly lower in BC cases than in controls (discovery round, 72 BC case and 24 controls, p = 2.61 × 10-9 adjusted for cell-type proportions; first validation round,338 BC case and 507 controls, p < 0.0001; second validation round,189 BC case and 189 controls, p < 0.0001). Compared to the highest quartile, the lowest quartile of cg27091787 methylation was associated with a more than 40-fold increased risk of BC (p < 0.0001). In addition to cg27091787, the decreased methylation of a 650 bp CpG island shore of HYAL2 was also associated with BC. Moreover, the expression and methylation of HYAL2 in leucocytes were inversely correlated (p = 0.006). To note, the BC-associated decreased HYAL2 methylation was replicated in T cell fraction (p = 0.034). The cg27091787 methylation level enabled a powerful discrimination of early stage BC cases from healthy controls (area under curve (AUC) = 0.88), and was also robust for the detection of BC in younger women (AUC = 0.87). CONCLUSIONS Validating the epigenome-wide study by independent cohorts, we have revealed a strong association between decreased HYAL2 methylation in peripheral blood and BC. Our results have given an answer to the debate on the origin of BC-associated differential methylation in blood, which is not only because of the change of cell type proportions, but more importantly due to altered methylation in specific blood cell fragments, like in T cells. And thus, we provide a promising blood-based marker for the detection of early BC.