Anorexia nervosa (AN) is a syndrome characterized by a distorted body image and morbid fear of obesity. Restrictive eating and compensatory behavior in the form of excessive exercising or vomiting result in malnutrition. Repeated revisions of the diagnostic criteria in recent decades have by definition resulted in an increase in prevalence. The disease has the highest mortality among all psychiatric disorders. The majority of deaths is related to malnutrition and its consequences. There is no effective evidence-based treatment. At least 25% remain chronically undernourished and have a long course with prolonged hospitalizations in both psychiatric and medical units, social psychiatric institutions and - support, early retirement and premature death. The etiology remains unknown and it is unclear which of the psychological symptoms that are a consequence of malnutrition, and which ones are premorbid or comorbid.