Hyperkalemia is commonly seen in hemodialysis patients and is a potentially life-threatening condition. Serum potassium concentration greater than 10 mEq/L is defined as extreme hyperkalemia. Emergent treatment of hyperkalemia is made with intravenous calcium, insulin-dextrose solutions, diuretics, gastrointestinal cation-exchange resin and dialysis. We present here a 52-year-old chronic hemodialysis patient with extreme hyperkalemia and cardiac arrhythmia. She was treated with emergent hemodialysis. Her abnormal laboratory findings and electrocardiography recovered. Further examinations suggested insufficient hemodialysis treatment as the possible cause of hyperkalemia.