A 68-year-old woman was hospitalised because of generalised weakness and development of confusional state, related to severe hyponatremia, probably due to an eating disorder with malnutrition. During the first days of hospitalisation the patient eats surprising large amounts of food. The worsening of the confusional state in spite of normalisation of natremia, and the progressive development of anemia and thrombocytopenia, leads to the discovery of a new onset severe hypophosphatemia. The rapid fall in plasma levels of phosphorus, magnesium and potassium are the main futures of the refeeding syndrome. Its clinical manifestations are neurological, muscular, haematological and renal. The development of this syndrome is associated with a high mortality. The refeeding syndrome is seen when carbohydrates are introduced after a period of malnutrition. Identification of patients at risk for the refeeding syndrome (anorexia nervosa, chronic alcoholism, chronic malnutrition, elderly patients, oncology patients), the introduction of cautious progressive nutrition and the careful monitoring of vital signs, electrolytes levels and fluid balance, allows to prevent morbidity and mortality of this syndrome.