Leptin is a hormone secreted mainly by the adipose cells with a primary role in the regulation of body weight by establishing a feedback loop between the energy reserves and the hypothalamic centers that control food intake. Recent data suggest that, in addition, leptin interacts with other endocrine systems to provide critical information about the size of the fat stores, acting as a permissive factor that allows the triggering of energy-demanding situations, as the onset of puberty and the reproduction, only when the size of the fuel reserve is large enough to guarantee its success. In addition, leptin appears to play a role during pregnancy and lactation, as it is produced by the placenta and is present in maternal milk. The fact that leptin levels are always higher in females, even after correcting for body fat content, suggests that the interaction between the adipose tissue and the reproductive system is modulated in a different way in males and females by androgenic and estrogenic hormones. In fact, adipose tissue samples taken from male donors are completely refractory in vitro to the action of both estrogens and androgens. On the contrary, dihydrotestosterone, androstenedione and dehydroepiandrosterone-S are potent inhibitors of leptin secretion, while estradiol induces a strong stimulation in adipose tissue taken from women. Testosterone is devoid of activity in either gender.