Introduction: Breast cancer (BC) is the main cause of death from cancer in women in Argentina. Therefore, immunological studies of the mechanisms involved in this pathology are relevant. Objectives: To evaluate the expression of Treg cell marker genes in peripheral blood (PB) from breast cancer patients and women free of disease and characterize the tumor samples. Materials and methods: PB and tumor samples from 41 BC patients and PB from 10 women free of disease (controls) were evaluated. Total RNA and cDNA were obtained from lymphocytes. FOXP3, CTLA-4, TNFR-2, and LAG-3 Treg markers gene expression was evaluated by PCR and qPCR. Clinical and histopathological data of BC patients were also registered. IDO, Foxp3, CD4, CD8, and CD45R0 protein expression were studied in tumor samples by immunohistochemistry. Results: In BC patients, 76% (31/41) of samples expressed FOXP3, while 100% (10/10) of control samples expressed this transcription factor. Besides, CTLA-4, TNFR-2, and LAG-3 expression was evaluated by PCR in selected samples from 22 BC patients and in controls. Pathological samples presented a differential gene expression profile in contrast to control samples. All the genes were expressed in BC samples, while the control samples only expressed FOXP3 and LAG-3. Subsequently, the expression levels of the genes commonly expressed, both in BC and control samples, were evaluated by qPCR. The expression levels of FOXP3 and LAG-3 were significantly higher in the BC samples compared to the controls (p<0.01). By IHC, 40% of samples showed Foxp3 expression, while 38% expressed Foxp3 in Tumor-infiltrating lymphocytes (TILs). Eighty-eight percent of samples expressed CD45R0+ TILs; 76%, CD8+ and 93%, CD4+ TILs. IDO was expressed in 54% of tumor samples. Conclusions: These results show the relevance of the study of immunological markers in PB liquid biopsies and tumor samples to evaluate the immunological status of BC patients with potential therapeutic implications.