Tension pneumothorax during one-lung ventilation is an insidious, life threatening emergency. Even though early diagnosis and rapid intervention are necessary to prevent mortality, treatment may be difficult because of the clinical diagnosis-mediastinal shift, hypoxemia, hypotension and increased airway pressure may be confusing in the operative setting. We present here the case of a patient who underwent right thoracotomy with one-lung ventilation and the patient developed left tension pneumothorax with resulting cardiac arrest.