This study involves 381 patients who received an implantable cardioverter defibrillator (ICD) for management of serious ventricular dysrhythmias and were followed forup to 9 years. The device was effective in preventing sudden cardiac death (SCD), with survival rates of 97% at 3 years and 94% at 5 years. Complication rates were low. Two hundred and twenty-five patients received at least one shock; 2.3 shocks per patient year were roceived. Shocks were categorized as appropriate, indeterminate, or inappropriate. Definitions of these categories and their occurrence are discussed and the characteristics and experiences of these patients are described.