Tumour necrosis factor-alpha (TNF) is a pleiotropic cytokine involved in many metabolic responses in both normal and pathophysiological states. In spite of the fact that this cytokine (also known as "cachectin") has been related to many of the metabolic abnormalities associated with cachexia, recent studies suggest that TNF may also have a central role in obesity modulating energy expenditure, fat deposition and insulin resistance. This review deals with the role of TNF in the control of fat mass and obesity.