Methylation is an epigenetic alteration proved to be involved in many disease processes including cancer. This change affects mainly gene promoters and repetitive sequences in genome. Long Interspersed Nuclear Element-1 (LINE-1) is a family of retrotransposons - repetitive elements that modify gene activity and can themselves be targeted by epigenetic mechanisms. LINE-1 methylation level is a surrogate marker for global methylation. In many conditions this parameter is found to be altered not only in affected cell groups, but also throughout other tissues. The aim of our study was to compare LINE-1 methylation pattern in DNA extracted from blood of the patients with benign and malignant breast tissue. In addition, we investigated correlation of LINE-1 methylation in blood and tissues of same patients and relationship of all variables with histopathologic and phenotypic characteristics of tumors. Patients with biopsy-proved ductal invasive carcinoma of breast and no preoperative chemo/radiotherapy were chosen for the study group. Another pool of patients with various benign breast lesions represented controls. Blood samples from both group members were collected preoperatively. Tumor tissue sections were processed for pathology report and part of remaining tissue was used for methylation study. LINE-1 methylation level was quantified using ELISA-based assay. It was analyzed in combination with histologic and phenotypic tumor parameters and compared between different tissues and different study groups. LINE-1 was found to be significantly hypomethylated in breast cancer tissue compared to blood. Blood samples of patients with malignant tumors showed slightly lower methylation level, than samples obtained from control group members. Lymphovascular invasion was the only aggressiveness-determining factor that was found to be at least weakly correlated with LINE-1 hypomethylation in blood. We can conclude, that global hypomethylation measured by LINE-1 methylation level is significant in tumor tissue. But there is no significant difference between LINE-1 methylation levels in blood of patients with benign and malignant breast tumors; therefor LINE-1 hypomethylation in blood cannot be used as a marker for early tumor detection. Neither is it valid for determination of tumor behavior.