In addition to diabetes and cardiovascular diseases, epidemiological evidence demonstrates that people who are obese or overweight are at increased risk of developing cancer - colon, breast (in postmenopausal women), endometrial or kidney cancer being among the most frequent. In addition to the increase in tumor occurrence, obesity also affects tumor prognosis, especially in breast and prostate cancers. In breast cancer, obesity is associated with reduced survival and increased recurrence independent of menopausal status. Host factors seem to contribute to the occurrence of tumors exhibiting an aggressive biology defined by advanced stages and high grade. Mature adipocytes are part of the breast cancer tissue and as highly endocrine cells susceptible to profoundly modify breast cancer cell behavior. Tumor progression has recently been recognized as the product of an evolving crosstalk between tumor cells and the surrounding 'normal' cells. We propose that such a bidirectional crosstalk exists between breast cancer cells and tumor-surrounding adipocytes, and that the tumor-modified adipocytes (or cancer-associated adipocytes) are key actors in tumor progression. The positive contribution of cancer-associated adipocytes into tumor progression might be amplified in obese women and explains at least in part the poor prognosis observed in this subset of patients.