BACKGROUND Refeeding syndrome is a complex metabolic disorder that develops following rapid nutritional administration after a long period of undernutrition. The onset mechanism involves intracellular transport of phosphorus, potassium, and water, in association with rapid glucose administration. The resulting hypophosphatemia is extremely dangerous and can cause severe heart failure and fatal arrhythmia. We successfully used extracorporeal cardiopulmonary support to manage a case of refeeding syndrome that occurred during the course of treatment of diabetic ketoacidosis. There are only a few reports of the use of cardiopulmonary support for the treatment of refeeding syndrome. CASE REPORT A 72-year-old man was admitted to the hospital for treatment of diabetic ketoacidosis. Despite receiving insulin and nutrition therapy, QT prolongation and ventricular fibrillation appeared on the electrocardiogram. Although coronary angiography was performed in consideration of the possibility of ischemic heart disease, no significant stenosis of the coronary arteries was identified. Due to persistent hypotension and recurrence of ventricular fibrillation, extracorporeal cardiopulmonary support was commenced in the ICU. His serum phosphorus level showed a marked decrease on his first day in the ICU, for which daily replacement therapy was administered during his ICU stay. No fatal arrhythmia developed thereafter. He was weaned off extracorporeal cardiopulmonary support on the fourth day of his ICU stay and was subsequently discharged from the hospital. CONCLUSIONS We suggest vigilant monitoring of electrolytes, including phosphate levels, in diabetic ketoacidosis patients, and active circulatory support, as required, in patients with refeeding syndrome.