Obesity is a worldwide problem which impacts the risk and prognosis of some of the more common forms of cancer, including breast cancer in post-menopausal women. As the basis for understanding the potential mechanisms of obesity and cancer relationship has advanced, a number of new hypotheses have emerged. The adipocytokines are a complex group of biologically active polypeptides. Leptin is a growth hormone, secreted by adipose tissue, whose levels are normally elevated in obese individuals and may have a promoting effect on carcinogenesis and metastasis of breast cancer, possibly in an autocrine manner. Leptin interferes with the insulin signaling pathway and in type 2 diabetes plasma leptin levels are found to be correlated with the degree of insulin resistance, a relationship independent of body mass. This relationship might provide a mechanistic explanation for promotion potential.