An emergency left frontotemporal craniectomy with direct neck clipping and hematoma removal was performed in a 36-year-old man with a ruptured left middle cerebral artery aneurysm and sylvian hematoma. Because of severe brain swelling postoperatively, we induced barbiturate coma therapy to treat his intractable brain swelling. He had an initial loading dose of sodium thiopental (5 mg/kg) followed by continuous infusion of sodium thiopental (5 mg/kg/hour). The lowest potassium concentration recorded during the barbiturate coma therapy was 1.1 mmol/L; necessitating treatment with cardiac massage, epinephrine, and atropine because of asystole and severe bradycardia. However, he did not recover from cardiac arrest. We present here a case of cardiac arrest due to severe life threatening hypokalemia that occurred during barbiturate coma therapy.