A 60-year-old female had sudden onset of syncope. The emergency service noticed that she suffered cardiopulmonary arrest (ventricular fibrillation: VF). After defibrillation in the ambulance, she was transported to our emergency department. Electrocardiography monitoring showed QT prolongation. Serum potassium level was extremely low at 1.8 mEq/l. Although potassium and lidocaine were administered, it was difficult to maintain appropriate electrolyte balance and prevent VF after admission, so temporary overdrive pacing was required. She was diagnosed as having primary aldosteronism after laboratory and imaging examinations. VF was otherwise uncontrollable so a cardioverter defibrillator was implanted on the 24th hospital day. Laparoscopic adrenalglandectomy was performed about 1 month later. After the surgery, serum potassium level remained at an appropriate level without medication. No severe neurological deficits were found at discharge from our hospital.