The authors found higher levels of CSF opioid activity, determined by radioreceptor assay, in patients with anorexia nervosa who were severely underweight than in 1) the same patients after weight restoration and 2) normal controls. Another group of patients who had chronic anorexia nervosa but were not severely underweight had normal levels of CSF opioid activity. Endogenous opioid systems have been shown to be related to eating behavior and metabolic regulation in animals. The association between decreased weight and increased CSF opioid activity observed by the authors may be a compensatory response to weight loss or may be etiologically related to anorexia nervosa.