Low body weight and rapid unintentional weight loss are highly predictive of mortality and morbidity in the elderly population. Weight loss is frequently reported in elderly patients. Acute and chronic diseases are leading causes of involuntary weight loss. Whereas physical disease probably accounts for a majority of cases of involuntary weight loss, psychiatric disorders such as dementia and depression also may result in severe nutritional deficiencies. Additional physiological, psychological, and social factors may affect food intake and body weight. Changes in body composition include loss of height and lean body mass and lower basal metabolic rate. Energy requirements decrease because of the lower basal metabolic rate and reduced physical activity. These low energy requirements make it more difficult for the elderly to obtain adequate amounts of required nutrients. Food intake regulation, taste, and olfactory sensitivity may be altered. As 50% of Americans have lost all of their teeth by age 65, chewing problems are often present. Other factors that contribute to poor nutritional status include alterations in the gastrointestinal tract, functional disabilities, lowered socioeconomic status, and social isolation. Finally, because of the increase in both physical and psychiatric disease, the elderly are major users of prescription drugs. Drug/nutrient interactions can result in anorexia and weight loss. The findings indicate that factors causing unintentional weight loss are highly interrelated and difficult to separate. Health care professionals must monitor body weight in elderly persons and carefully evaluate any cases of rapid, unintentional weight loss to prevent further deterioration of health status.