To exclude inappropriate shocks and to verify that therapy delivered to an asymptomatic patient was appropriate, only patients with an ICD with interval (Ventak PRx Model 1700, 1705, 1710 and 1715, CPI, Minneapolis, Minnesota) or electrogram recording capability (Cadence Model V100, V100B, V100C, Ventritex, Sylmar, California, and Ventak PRx Model 1715, Guidant, Indianapolis, Indiana) were included. The records of 125 consecutive patients who had undergone implantation using one of these ICDs for symptomatic ventricular tachycardia (VT) or cardiac arrest and who were followed by one of the authors were examined. Patients in whom an ICD was implanted prophylactically were excluded from the study. Baseline data for each patient included gender, age, type of heart disease, history of myocardial infarction, history of congestive heart failure, past history of coronary artery bypass grafting, LVEF, electrogram QRS duration and signal-averaged electrogram. Type of presenting arrhythmia, cardiac arrest (CA) or SMVT was determined. For patients with SMVT, the arrhythmia cycle lengths and symptoms were noted.