AbstractA patient with a loud intermittent midsystolic click presented a problem in differential diagnosis between mitral valve prolapse (intracardiac origin of the click) and left pneumothorax (extracardiac origin). External phonocardiography performed at the time of cardiac catheterization revealed that this loud midsystolic click disappeared whenever a catheter was positioned across the mitral valve. It reappeared whenever the catheter was removed from the transmitral position. Selective left ventriculography confirmed the diagnosis of mitral valve prolapse.