Background Recent development of a prototype single-lead unipolar transvenous defibrillator offers the possibility of device implantation with the ease of a permanent pacemaker. Lowering defibrillation energy requirements would allow for a further reduction in defibrillator generator size and enhance the feasibility of pacemaker-like placement. However, if achieving a lower defibrillation energy requires placing additional intracardiac leads, the potential advantage of a smaller generator may be offset by the disadvantages of a more complex lead system. The purpose of this study was to compare defibrillation energy requirements of a single-lead unipolar defibrillator with a three-electrode system employing an additional lead in the coronary sinus..4). Using either defibrillation system, all patients were successfully defibrillated by <24 J and over half of patients by <10 J.Conclusions A unipolar transvenous biphasic defibrillation system is an effective means of treating ventricular fibrillation. The added complexity of additional leads is not offset by any significant improvement in defibrillation efficacy or energy requirements. Given the simplicity and effectiveness of a single-lead system coupled with a small generator, placement of defibrillation systems may now approach the ease of pacemaker implantation. (Circulation. 1994;89:2641-2644.)