Body composition studies were performed in 33 patients suffering from the acquired immunodeficiency syndrome (AIDS). Studies included measurements of total body potassium, fat, and total body and extracellular water volumes plus serum retinol binding protein concentration, iron binding capacity, and albumin concentration. AIDS patients were underweight (p less than 0.001) and were depleted of potassium (p less than 0.001) with the lowest values occurring in patients close to death at the time of study. Body fat contents also were reduced. Intracellular water volumes were decreased in the AIDS patients (p less than 0.001) with a relative increase in extracellular water volume (p less than 0.001). Serum protein concentrations were decreased in the AIDS patients. Longitudinal studies did not demonstrate tissue repletion in patients with AIDS, despite apparent clinical stability. These studies demonstrate that severe, progressive malnutrition occurs in patients with AIDS. If malnutrition can be shown to have a deleterious effect upon the disease course, therapy of malnutrition may play an important role in the treatment of this disorder.