Obesity is a rapidly increasing health problem in all developed countries. Overweight rarely occurs in isolation but as part of a complex pattern of metabolic abnormalities ("metabolic syndrome" or "syndrome X") consisting of hyperlipidemia, hypoalphalipoproteinemia, type II diabetes and atherosclerosis. The disorder is considerably influenced by genetic, behavioural and nutritional factors. Recent data indicate that a group of closely related nuclear receptors, the peroxisome proliferator-activated receptors (PPARs), may be involved in the metabolic changes ultimately leading to obesity. This review summarises the latest developments in the PPAR field, with particular emphasis being placed on the physiological function of PPAR alpha during various nutritional states, and the possible role of PPAR alpha in obesity.