Leptin is a recently discovered hormone which is involved in the regulation of body weight. It provides a molecular basis for the lipostatic theory of the regulation of energy balance. White adipose tissue is the main site of leptin synthesis and there is some evidence of ob gene expression in brown fat. Leptin seems to play a key role in the control of body fat stores by coordinated regulation of feeding behaviour, metabolic rate, autonomic nervous system regulation and body energy balance in rodents, primates and humans. Apart from the function of leptin in the central nervous system on the regulation of energy balance, it may well be one of the hormonal factors that signal the body's readiness for sexual maturation and reproduction to the brain. During late pregnancy and at birth when maternal fat stores have been developed leptin levels are high. Leptin could then be a messenger molecule signaling the adequacy of the fat stores for reproduction and maintenance of pregnancy. At later stages of gestation leptin could signal the expansion of fat stores in order to prepare the expectant mother for the energy requirements of full term gestation, labour and lactation. This overview focuses on those topics of leptin research which are of particular interest in reproductive medicine and gynecology.