- Background: Implantable cardioverter defibrillators (ICDs) reduce mortality in patients implanted for primary and secondary prevention of sudden cardiac death. Data on the incidence of appropriate ICD therapies in primary vs. secondary prevention are limited. Objectives: To compare ICD therapies and mortality in primary vs. secondary prevention of sudden cardiac death. Methods: We conducted a retrospective analysis of 581 consecutive patients receiving an ICD for primary (66%) or secondary (34%) prevention indications. Results: During long-term follow-up, 29% of patients implanted for secondary prevention received appropriate ICD therapy vs. 18% implanted for primary prevention. However, the overall 7 year mortality rate was not significantly different between the two groups (26.9%, P = 0.292). Multivariate analysis showed that patients implanted for primary prevention had a significantly lower risk of appropriate ICD therapy even after adjustment for age, left ventricular ejection fraction < 0.35 and chronic renal failure (HR 1.63, 95%CI 1.10–2.41, P = 0.015). Conclusions: Patients implanted for secondary prevention were more likely to receive appropriate ICD therapy, with a significantly shorter time period from ICD implant to the first therapy. However, all-cause mortality was comparable between primary and secondary prevention groups.