Obesity is not a single disease, but a variety of conditions resulting from different mechanisms and associated with various types and degrees of risks. To determine who should lose weight, how much weight should be lost, and how to undertake weight loss, the following types of information are needed: personal-demographic data, developmental patterns, family history, energy balance, body composition/fat distribution, psychological/behavioral measures, endocrine/metabolic measures, complications and associated conditions. Weight reduction should be undertaken by women with morbid obesity, with complications secondary to the obesity, with a strong family history of conditions associated with obesity, or with increased abdomen:hip ratios. In contrast, women who have excess weight localized in the hips and thighs and no personal or family history of associated conditions may not benefit from dietary restriction. Low calorie diets result in adaptive changes, "designed" to prolong survival in the face of famine. These include changes in water balance, metabolic rate, and appetite. Metabolic rate declines, allowing the individual to burn fewer and fewer calories. Each time a woman diets she tends to lose weight less rapidly than the time before. "Restrained eating" predisposes binge eating. Indeed, bulimia rarely occurs in the absence of prior caloric restrictions. Current medical definitions of obesity do not consider these nuances. Existing definitions "over-diagnose" obesity in women, in general, and in older women and nonwhite women, in particular. For example, by existing standards, more than 60 percent of black women more than 45 years of age are considered obese. In contrast, the health risks of similar degrees of obesity are substantially greater for men than for women. Part of the problems lies in the fact that many women have pear-shaped fat distribution,a pattern which is not associated with increased health risks.Current cultural definitions of obesity for women distort the picture even further. In the past 20 years,there has been a progressive decline in the weight-for height of such "culture models" as Playboy centerfold subjects and Miss America contestants. Attempting to achieve such low weights predisposes women to an endless cycle of dieting and regaining, and contributes to the growing problems of eating disorders, including anorexia nervosa and bulimia.