Aims Cardiac resynchronization therapy (CRT) improves symptoms, left ventricular function, and survival in patients with heart failure (HF) and wide QRS. The benefit of adding implantable cardioverter-defibrillator (ICD) backup is debated. We analysed the long-term outcome of patients with HF due to ischaemic cardiomyopathy (ICM) or non-ischaemic cardiomyopathy (NICM) treated with a CRT device with or without defibrillator backup. Methods and results In this observational study, consecutive patients with an ejection fraction ≤35% and QRS width ≥120 ms receiving a CRT device at Aarhus University Hospital, Denmark from 2000 to 2010 were included. Baseline characteristics were retrieved from patient files and survival data were obtained from the Danish Civil Registration System. The primary outcome was all-cause mortality. The effect of ICD backup was estimated using Cox proportional hazards model, and the multivariate analyses were adjusted for a priori selected variables. We included 917 HF patients, 427 with NICM, and 490 with ICM. Median follow-up was 4.0 years. Adjusted hazard ratio (aHR) for all-cause mortality was 0.76 [95% confidence interval (95% CI), 0.60-0.97; P = 0.03] in all patients; 0.96 (95% CI, 0.60-1.51; P = 0.85) in patients with NICM, and 0.74 (95% CI, 0.56-0.97; P = 0.03) in patients with ICM. In patients with NICM, ICD backup seemed to be associated with improved survival among non-responders to CRT (P = 0.08), but not among responders (P = 0.61). Conclusion Adding an ICD backup is associated with better survival in CRT recipients. This effect was evident among patients with ICM, but not in patients with NICM. Published on behalf of the European Society of Cardiology. All rights reserved.