The mechanism involved in body mass regulation in humans includes genetic, environmental, and behavioural factors. Human obesity is usually associated with a positive energy balance. Genetic studies in obese mice have revealed the Ob. gene, its products leptin and the leptin receptor to be important factors in the regulation of both appetite and energy expenditure. Leptin is a 16-kilodaltons adipocyte-derived hormone -which circulates in the serum as the free and bound forms. The leptin serum level reflects the amount of energy stored in adipose tissue. Leptin acts through the leptin receptor, -which belongs to the cytokine - receptor family. In rodents as well as in humans, homozygous mutations in genes encoding leptin or the leptin receptor cause early-onset morbid obesity, hyperphagia, and reduced energy expenditure. Recent studies have demonstrated that Ob. gene expression is increased in human obesity. However, mutations of Ob. gene present in the mouse are rare in the human population.