The first 50 websites identified on searching the Internet for 'weight loss diets' were assessed systematically and their content compared with published clinical guidelines for management of obesity. The relevance and quality of the sites varied enormously. Only 3 confined themselves to sound dietary advice. Most promoted dietary supplements or other 'slimming aids', often of uncertain composition and based on dubious physiological principles. Potential hazards--for example, those of very low calorie diets--were rarely highlighted and certain regimens on offer were potentially dangerous.