Weight loss among elderly patients is a common clinical problem that may herald numerous medical and psychosocial disorders. Up to 50% of patients claiming weight loss will not have their complaint corroborated by medical records or family members. Since modest weight loss may occur as a physiologic change with advancing age, the practicing physician must have a working definition of pathologic weight loss that triggers an appropriate diagnostic evaluation. After a careful initial history, physical exam, and a limited laboratory test battery, physicians will identify most patients with physical causes for weight loss. Like many other geriatric syndromes, weight loss may require the identification and correction of multiple contributing factors.