Introduction: Polycystic ovary Syndrome (PCOS) is one of the most prevalent endocrinology disorders in women, in whom the state of systemic inflammatory cytokines, especially TNF-α is the main reason for immunological disturbances. Some PCOS manifestations such as infertility, hyperandrogenism, obesity and chronic inflammation are considered as risk factors for breast cancer. The risk of developing breast cancer in women with PCOS is being investigated in some epidemiological studies. In this research, the ability of peripheral blood mononuclear cells (PBMCs) of women with PCOS to develop antitumor response was studied and evaluated using an experimental co-culture approach between PBMCs and breast tumor cell lines. Materials and Methods: PBMCs were isolated from 50 heparinized venous blood samples (patient and healthy groups) by density gradient centrifugation byficoll. Breast cancer cell lines (MDA-MB-468 and MCF-7) were incubated as two target cells and were cultured adjacent to PBMCs in a transwell co-culture system. At different time intervals (48 and 72 hours) after co-culture, the proliferation rate of the effectors cells was evaluated by the BrdU cell proliferation assay. Determination of T CD3+CD8+ lymphocytes was determined by flow cytometry. Results: The proliferation of PBMCs after 48 hours of co-culture with MDA-468 (P=0.002) and MCF-7 (P=0.021) was significantly higher in the PCOS group compared to healthy controls. No pronounced differences were observed in T CD3+CD8+ cell numbers between the PCOS group and healthy controls (P>0.05) although T CD3+CD8+ percentage increased after 72 hours of co-culture in most samples. There was no statistically significant difference between MDA-468 and MCF-7 co-cultures in any of the tests. Conclusion: The stimulation threshold for mononuclear cells was reduced in women with PCOS. Differences between proliferation responses of PCOS and control groups may be caused by a chronic inflammatory condition in these patients.