The refeeding syndrome is frequent and potentially deadly, still it is underdiagnosed. It is defined by clinical and biological manifestations that are seen upon refeeding of malnourished patients. It is the consequence of the transition from catabolism to anabolism. Ions intracellular shift caused by insulin and B1 vitamin deficiency are fundamental in the development of this syndrome. Riskconditions are well summarized by the NICE criteria. To avoid refeeding syndrome, it is fundamental to find and correct any electrolytic deficiency and to give thiamine before starting a slow and progressive oral, enteral or parenteral refeeding.