The use of implantable defibrillation systems in patients with cardiac arrest has resulted in lower mortality than expected from studies of similar patients not receiving defibrillators. Nonthoracotomy lead systems have led to a decrease in operative mortality and lowered the cost of defibrif lator implantation, but these systems have a higher energy requirement for defibrillation than do epicardial ones. The recent introduction of single-lead systems and bipolar defibrillation pulses has simplified nonthoracotomy defibrillator implantation and improved defibrillation efficiency. A prototype unipolar, single-lead pectoral implant defibrillation system is described that may significantly improve the reliability, safety, and cost effectiveness of nonthoracotomy defibrillators. This and other improved nonthoracotomy systems may expand the indications for defibrillator implantation to prophylactic use in high-risk patients who have not yet experienced life-threatening ventricular arrhythmias.