One quarter to one third of Americans are overweight; as many as 40% of women and 24% of men are trying to lose weight at any given time; many have tried a variety of methods, such as diets, exercise, behavior modification, and drugs. In controlled settings, participants who remain in weight loss programs usually lose approximately 10% of their weight. However, one third to two thirds of the weight is regained within 1 year, and almost all is regained within 5 years. For many overweight persons, achieving and maintaining a healthy weight is a lifelong challenge. Successful weight loss improves several cardiovascular risk factors and diabetic control; effects on mortality are not clear. Several epidemiologic studies have found that weight loss is associated with increased mortality but the reasons for weight loss were not known. Survey data also confirm that many Americans who are not overweight, particularly young women, are trying to lose weight, which may have adverse physical and psychological consequences. Because of the importance of these issues, research on weight and on weight loss and control should assume a high priority on the nation's health agenda.