Background: Obesity, an increasingly prevalent health problem, is a major risk factor for recurrence and morbidity of breast cancer in postmenopausal women. Moreover, obesity increases therapy resistance and tumour escaping. To limit the obesity adverse effects, one of the health recommendations is regular physical practice. To clarify the physical activity effects on obesity and breast cancer development, it is important to consider the physical activity impact not only on metabolism but also on tumour immunity balance between pro and anti-tumour immune response. Objectives: The purpose of this study was to examine the influence of spontaneous physical activity, promoted by an enriched environment (EE), on systemic biological markers and on tumour immune response in C57BL/6 mice (33 weeks old) ovariectomized and fed with a high fat (HFD). Systemic and tissue biological assays evaluated energetic substrates (i.e. glucose, cholesterol, triglycerides) and signalling molecules (i.e. chemokines, cytokines, adipokines). Flow cytometric analysis permitted to phenotype immunity balance in terms of pro tumour cells (myeloid derivate suppressive cells (MDSC), regulator T lymphocytes (Treg)) and anti-tumour cells (Natural Killer (NK), cytotoxic T lymphocytes (Tc)). Immune trafficking between lymphatic tissue and tumour completed this approach. Results: Ovariectomized old mice feeding with HFD presented higher body weight and adiposity (fat mass: +5 ± 0.5g, p<0.05) associated with higher plasma levels for leptin, estradiol and glycaemia. This fat phenotype was associated with a decrease in spontaneous physical activity (-22 ± 5%, p<0.01). At the opposite, housing in the enriched environment reduced adiposity, leptin, estradiol and glycaemic levels and increased spontaneous physical activity of the mice (p<0.05). After tumour implantation, a significant decrease of spontaneous physical activity was observed in all groups, but less important in the enriched environment(-56 ± 10% vs -39 ± 10%, p<0.05). In addition, tumour growth was enhanced by HFD and linked to a worse survey. Housing in the enriched environment reduced tumour growth and increased the survey (p<0.05). HFD-tumour immune phenotyping presented an imbalance between pro- and anti-tumour cells in favour of pro-tumour cells. A significant increase of MDSC and Treg appeared (24.7 and 21.5% of the immune infiltrate) in contrast to a decline of Tc content (5.3%). Immune trafficking from spleen to inguinal lymph nodes reflected these tumour immunity modifications EE changed tumour immune phenotype, resulting in an equilibrate immune response with a higher Tc infiltration (19.9%) and a lesser MDSC and Treg content (11.1 and 1.4%). Conclusion: These data confirmed that HFD imbalanced the anti-tumour immunity response and enhanced fat mass, leptinemia and tumour growth. Interestingly, they highlighted the beneficial effect of spontaneous physical activity to limit these carcinogenesis disorder