Deficiency of the adipocyte-derived hormone leptin in ob/ob mice results in severe, early-onset obesity and infertility. Administration of leptin results in complete reversal of the phenotype, suggesting that leptin is needed for the development of puberty in rodents. In humans, mutations in the genes encoding leptin and the leptin receptor result in obesity syndromes and hypogonadotropic hypogonadism. I have shown that administration of recombinant human leptin results in the onset of puberty at an appropriate developmental age in human congenital leptin deficiency. This work suggests that leptin is a metabolic gate for the onset of puberty in humans. Leptin's actions may be mediated by central pathways and by direct action on peripheral organs.