Efforts have been focussed on the development of implantable cardioverter-defibrillator systems that reduce the size of the devices, simplify the methods of electrode implantation, and increase device longevity. The size of implantable units is predominantly determined by the capacitors and batteries, and this is dependent on the output energy requirements. The energy required for defibrillation and therefore the size of the devices can be reduced by utilizing optimal lead systems and waveforms. This report describes our experience with various shock waveforms in 86 consecutive patients undergoing implantation of a cardioverter-defibrillator with a nonthoracotomy approach. Table 1 provides information on the demographic characteristics of the study population, the various shock waveforms and lead configurations tested. In 20 patients, a bidirectional lead configuration was tested using simultaneous and sequential monophasic waveforms. In 18 patients, simultaneous monophasic and biphasic waveforms with a fixed pulse duration of 6.3 ms were employed utilizing a bidirectional lead system. In 31 patients, a undirectional lead configuration was used, and simultaneous monophasic and biphasic shocks with a constant tilt were delivered. In 17 patients, the defibrillation efficacy of biphasic and triphasic shock waveforms with a fixed pulse duration of 10 ms was evaluated. The study design was an open, randomized and prospective evaluation in all patients. Defibrillation threshold was determined in a randomized sequence using a step down protocol. The defibrillation threshold was defined as the lowest energy required for effective defibrillation on two occasions. In the 20 patients using a bidirectional lead configuration, there was no significant difference in defibrillation energy requirements between simultaneous and sequential monophasic shock waveforms.(ABSTRACT TRUNCATED AT 250 WORDS)