Leptin seems to play an important role in the generation and maintenance of immune responses. Leptin is a cytokine similar in structure to interleukin 2, an important T-cell growth factor. Energy balance and supply is increasingly being realised as an important factor in the survival and function of immune cells. Immune responses are intrinsically energy expensive and come at a cost to the responding organism. A fall in leptin concentration, as occurs in starvation, causes impaired cellular immune responses with proinflammatory and Th1 immune responses being particularly affected. Animal models of leptin deficiency show impaired cognate immune responses and are resistant to a variety of autoimmune diseases, mainly those dependent on intact T-cell immunity. The next step is to determine whether modulation of the leptin axis is therapeutically beneficial in a variety of autoimmune or infectious diseases.