Anorexia nervosa exhibits one of the highest death rates among psychiatric patients and a relevant fraction of it is derived from undernutrition. Nutritional and medical treatment of extreme undernutrition present two very complex and conflicting tasks: (1) to avoid "refeeding syndrome" caused by a too fast correction of malnutrition; and (2) to avoid "underfeeding" caused by a too cautious refeeding. To obtain optimal treatment results, the caloric intake should be planned starting with indirect calorimetry measurements and electrolyte abnormalities accurately controlled and treated. This article reports the case of an anorexia nervosa young female affected by extreme undernutrition (BMI 9.6 kg/m(2)) who doubled her admission body weight (from 22.5 kg to 44 kg) in a reasonable time with the use of enteral tube feeding for gradual correction of undernutrition. Refeeding syndrome was avoided through a specialized and flexible program according to clinical, laboratory, and physiological findings.