Paediatric departments are increasingly faced with adolescents with anorexia nervosa in severe nutritional and metabolic decompensation. Artificial nutritional support, often indispensable in the most critical phases of the course, requires careful evaluation of the clinical conditions at the time of hospitalization and equally punctual monitoring of blood chemistry variables during re-feeding. Prolonged reduced caloric intake and malnutrition, especially in its most extreme forms, can cause damage to various organs and systems and the refeeding syndrome must be kept in mind as a further comorbidity and aggravating factor of the patient's conditions. Through the description of a clinical case of particular complexity, some aspects related to the hospital treatment of anorexia nervosa are discussed.