Of 533 patients with implantable cardioverter defibrillator (ICD) implants, 40 who had malignant ventricular arrhythmias and functional class IV heart failure (mean age 65 +/- 7 years, mean left ventricular ejection fraction 0.24 +/- 0.08) were found to have a mean time to the first appropriate ICD therapy to death of 25 +/- 22 months and a mean actuarial survival of 64 +/- 5 months. Prolonged survival can occur, and appropriate therapy can be delivered, by an ICD in high-risk patients with functional class IV heart failure who have malignant ventricular arrhythmias.