Many clinicians and policymakers are concerned whether use of the implantable defibrillator (ICD) is justified in view of its high cost. Three randomized trials of the ICD have reported economic outcomes. Each trial found a large difference in cost between patients assigned to an ICD versus patients assigned to conventional therapy that persisted over three to six years of follow-up. Each trial also found better survival among ICD patients, and calculated ICD cost-effectiveness (CE) ratios between 27,000 dollars per life year added and 139,000 dollars per life year added. The variability in the cost-effectiveness ratios among trials is mainly due to variability in the years of life added by the ICD among the trials and, by extension, among patient subgroups. A rough rule of thumb is that the ICD will be economically attractive when it prolongs mean survival by six months or more, which is attainable in higher risk patient subgroups.