Refeeding syndrome (RFS) can be a severe and life-threatening complication of anorexia nervosa (AN) associated with electrolyte abnormalities and organ damage, and occurs with the transition from a prolonged catabolic to anabolic state, particularly with an overzealous nutrient supply. There is no unequivocal definition of RFS, although hypophosphatemia is recognized as a crucial factor in its pathogenesis. RFS can be responsible for cardiovascular complications, such as heart failure, left ventricular damage, and arrhythmias, because of different potential mechanisms: electrolyte imbalances, increased retention of sodium and liquids secondary to insulin secretion, and excessive fat emulsion supplementation. We report on the case of a 13-y-old male patient with severe AN in whom a delayed and reversible myocardial dysfunction was documented during cautious nutritional replenishment, even in the absence of serum electrolyte imbalances. Seven days after the inception of integrative enteral nutrition, heart failure was unexpectedly documented as follows: reduction in fraction ejection, presence of mild bilateral perimalleolar edema, and increased n-terminal prohormone of brain natriuretic peptide. A more pronounced water restriction protocol and delayed achievement of goal feeding rate, resulting also in lower sodium intake, were implemented to reduce cardiac overload with a full resolution of the complication in approximately 2 mo. Refeeding patients with AN could be complicated by heart failure despite cautious nutritional replenishment and regardless of electrolyte imbalance, even in a later phase of recovery. Therefore, strict adherence to recommendations for nutritional replenishment and close monitoring of cardiac function should always be considered when refeeding patients with AN.