Despite the current opinion that leptin can no longer be seen as a hormone which could be used therapeutically to prevent an energy surplus (it rather protects the organism for an energy deficit), leptin may still have an impact in clinical medicine. Leptin was shown to have several important functions. The pleiotropic properties of leptin include a regulatory function in the immune system. Reviewing the effects of leptin on different parts of the immune system reveals that the immune system is deregulated in an environment low in leptin. A strong reduction in leptin levels occurs in situations of starvation as seen after bariatric surgery. We postulate the hypothesis that the starvation-induced postoperative decrease of leptin is causative of the more serious course of complications observed after bariatric surgery.