A case of accidental exposure of a patient undergoing hemodialysis to a sodium hypochlorite solution is reported. A 61-year-old woman was completing a hemodialysis treatment when routine cleaning of the hemodialysis machine was started. Approximately two liters of undiluted sodium hypochlorite cleaning solution (Chlorox) was added to the dialysis bath, soaking the membrane fibers. For less than two minutes the Chlorox-soaked membrane was in contact with the blood returning to the patient. This accident led to massive hemolysis, hyperkalemia, cyanosis, and cardiopulmonary arrest. Hemolysis may have been caused by the hypertonic solution, rapid exothermic protein degradation, alkaline degradation, or another mechanism. The sudden rise and fall in the concentrations of serum electrolytes and subsequent hyperkalemia was the most probable cause for the cardiac arrest. Cardiopulmonary resuscitation was started, the patient was intubated, given oxygen, sodium bicarbonate, atropine, dopamine, and isoproterenol. Sodium thiosulfate 5 g was administered by a nasogastric tube approximately 25 minutes after the cardiac arrest as a neutralizing reducing agent. The patient's condition stabilized, and she recovered after a week of hospitalization. Cleaning solutions used in the routine cleaning of hemodialysis machinery represent potentially toxic agents. Hemodialysis procedures should ensure that cleaning and sterilizing solutions cannot accidentally come into contact with a dialysis machine that is still connected to the patient.