Patients receiving minimally symptomatic shocks from their implantable cardioverter defibrillators were studied prospectively using transtelephonic ECG loop monitoring. The time course to the first subsequent shock was evaluated. Twenty-nine consecutive patients who received a shock preceded by mild palpitations or no symptoms were given a transtelephonic ECG loop monitor and instructed to activate the monitor if a subsequent shock occurred. Kaplan-Meier analysis was used to quantitate the time to first shock during the study period. The point estimate +/- standard error of patients receiving a shock during the study period was 31% +/- 9% at 30 days, 41% +/- 9% at 60 days, and 60% +/- 9% at 120 days. The ECG was successfully transmitted in 7 of 13 patients who had shocks in the 60-day monitoring period, and demonstrated inappropriate shocks in 6 of 7. Determination of the cause of shock led to a change in subsequent management in all 7 patients. We conclude that the incidence of inappropriate shocks may be higher than estimated previously in patients with minimal symptoms prior to the shock. There are thousands of patients with implantable cardioverter defibrillators that have no storage function for treated tachycardias; transtelephonic ECG loop monitoring can determine the cause of implantable cardioverter defibrillator discharge in these patients, and the diagnosis is invaluable in their management.