Treatment with an ICD is the first-line treatment for survivors of sudden cardiac death. More recently, evidence accumulates that prophylactic ICD therapy may be beneficial for selected subgroups of patients after myocardial infarction. Particularly for future studies on the value of prophylactic ICD therapy, downsized devices are needed to allow easy pectoral implantation with a single lead configuration and featuring extended memory capabilities. Accordingly, this study assesses the clinical performance of a downsized fourth-generation ICD in 162 consecutive patients. All devices could be successfully implanted pectorally, in 96% with a single lead configuration with a low defibrillation threshold of 10.6 +/- 5.2 J. During a 3-month follow-up, 26% of the patients received ICD therapy. Twenty percent had appropriate therapy for ventricular fibrillation (n = 9) and VT (n = 23), which was effective in all cases. Of the 450 episodes of VT, 426 were terminated by antitachycardia pacing. Fourteen patients (9%) had inappropriate ICD therapy mainly due to atrial fibrillation or sinus tachycardia, which could be reliably diagnosed by the ICD stored intracardiac electrograms.