nowadays obesity can be considered a global epidemic illness, though it shows a long-term treatment failure index higher than 90%. research work on leptin, which is a peptide mainly produced by fat cells, lead to a breakthrough for medical obesity treatment. it has been found out that leptin - through control of its concentration, could be a marker of the energetic status and depot of fat amounts in cells as well. this occurs by acting in a feedback circuit which function is to maintain body weight stable - this independently of food intake variations. hence, if this circuit presents failures, as i.e. low response to high leptin concentration, patients may become obese. this phenomenon is very common and lead to the "leptin resistance thesis". the aim of this review is to explain leptin's physiological function as an hormone that "prevents" weight loss because physiological repercussion of leptin concentration reduction seems to be more intense than when it increases - at least in human beings. therefore what has been called - by definition - leptin resistance - seems to be just the result of a concentration which causes a limited physiological action.