Publisher Summary This chapter discusses the causes and diagnosis of loss of weight. Loss of weight is the result of inadequate alimentation whether from lack of food, poor appetite, or inability to swallow or to retain. It is important to distinguish between those patients who lose weight despite normal food intake and those whose calorie intake is diminished. In children, the commonest causes are malnutrition from injudicious feeding and gastrointestinal disease; in adults, when loss of weight is considerable, with no definite physical signs and little relevant history, it is first thought of malignant disease, a chronic infection such as tuberculosis, hyperthyroidism, diabetes mellitus, anorexia nervosa, or some other psychiatric disturbance. Young persons may lose weight as the result of change of surroundings, for instance, from active outdoor school life to work in a city office; care and anxiety, sorrow, disappointment in love, too strenuous a life of pleasure, irregularity of meals, over-long hours of work are familiar causes of what at the time may appear to be ominous loss of weight. In some cases, an accompanying loss of appetite and inadequate food intake supply a plausible explanation.