Underweight is a key symptom in anorexia nervosa. In this review we summarize recent findings pertaining to weight regulation in this eating disorder. The observation that a body mass index below 13 kg/m2 upon admission for inpatient treatment is associated with a high mortality rate and chronic persistence of underweight is of obvious clinical relevance. A lowered leptin secretion, which results from the weight loss, is presumably of major importance for the development of amenorrhea. We discuss findings pertaining to a reduced body weight in other psychiatric disorders during adolescence in the light of Kretschmer's findings related to body frame and psychopathology.