We describe here a 87-year-old male who came to the emergency room with the chief complaints of dyspnea and chest pain. His electrocardiogram showed a wide QRS tachycardia with a heart rate of 140 beats/min, a left bundle branch block pattern and low voltage in leads I, aVL, V5 and V6. A long strip showed a premature ventricular complex, and the sinus beat just after the extrasystole showed P waves which were positive in leads I, II, III and a VF similar in shape to those in sinus rhythm. Therefore, a diagnosis of sinus tachycardia with tachycardia dependent left bundle branch block was made. The low voltage in the left lateral leads was ascribed to pneumothorax of the left lung. When a wide QRS tachycardia is encountered in an emergency situation, marked sinus tachycardia with coexistent bundle branch block or aberrant ventricular conduction should be taken into consideration.