The leptin, protein of 146 amino acids, produced gene ob (chromosome 7) acts via the hypothalamus on the control of energy balance. It is implied in the adequacy between the food intake and the energy expenditure, and maintains stable body mass. We studied leptine's diurnal rhythm in old person of more than 75 years in institution, according to its state of nutrition and the undercurrent events likely to modify it. The study subjects were divided in two groups according to BMI < or superior 20 kg/m2. In the group of the patients with BMI higher than 20 kg/m2, there exists a correlation of the variation of the plasma leptin levels according to the BMI. At the subjects having a BMI lower than 20 kg/m2, the plasma leptin level is higher than the awaited value and the Leptin/BMI ratio is increased significantly compared to the subjects having a BMI higher than 20 kg/m2 (p < 0,05). Reduced nutritional status has largely exceeded capacities of adaptation to the nutritional needs and the capacity of response to the stress via the leptin is very disturbed.