Obesity--defined by a body mass index above 30 kg per m2--is a major problem for affluent nations. Its prevalence is higher in North America than in Europe--between 9% and 12% of the population. Reduced energy expenditure from exercise or metabolism or both may be an important contributory factor in the development of obesity because of a failure to reduce food intake sufficiently to maintain energy balance. A high ratio of abdominal circumference relative to gluteal circumference carries a twofold or greater risk of heart attack, stroke, hypertension, diabetes mellitus, gallbladder disease, and death. The effect of increased quantities of abdominal fat is greater than that of a similar increase in total body fat on the risks of ill health associated with obesity. Genetic factors appear to contribute about 25% to its etiology.