Implanted cardioverter defibrillators (ICDs) are very effective in the treatment of life-threatening ventricular arrhythmias. Among patients with severe left ventricular (LV) dysfunction, ICDs can defibrillate at energies similar to those in patients with well-preserved LV function; the perioperative mortality rate with transvenous systems is low, even in patients with a low ejection fraction (EF). In patients whose EF is < 30%, sudden death and total mortality rates are relatively low. However, at most only 50% of deaths in patients with low EFs are sudden, and life in these patients may be only slightly prolonged by successful defibrillator shocks. The true value of ICDs in these patients can be determined only by comparison with optimal medical therapy in randomized controlled studies. The Canadian Implantable Defibrillator Study randomly assigns patients with ventricular fibrillation or ventricular tachycardia to ICD or amiodarone. Current patient profiles show that most of these patients have poor LV function (mean EF, 33.6%), and most receive optimal medical therapy. Results of this and other studies will clarify the benefits from ICDs in these patients with severe heart disease.