Large-scale data on outcomes with cardiac resynchronization therapy with defibrillator in patients with diabetes mellitus are limited. We compared outcomes after cardiac resynchronization therapy with defibrillator implantation among patients with heart failure who have diabetes mellitus versus those without diabetes mellitus. Survival curves and covariate adjusted hazard ratio (HR) or odds ratio were used to assess the risks for death, readmission, and device-related complications by diabetes mellitus status among 18 428 patients at least 65 years old receiving cardiac resynchronization therapy with defibrillator from the National Cardiovascular Data Registry, implantable cardioverter-defibrillator registry between 2006 and 2009, with up to 3 years of follow-up. Accounting for differences between groups, compared with those without diabetes mellitus (n=11 345), patients with diabetes mellitus (n=7083) had a higher risk of death both at 1 year (HR, 1.16 [95% confidence interval (CI), 1.05-1.29]; P=0.0037) and 3 years (HR, 1.21 [1.14-1.29]; P<0.001) after device implantation and higher risks of all-cause readmission (sub-HR, 1.16 [1.11-1.21] at 1 year; P<0.0001; sub-HR, 1.15 [1.11-1.19] at 3 years; P<0.0001) and heart failure-related readmission (sub-HR, 1.18 [1.09-1.28] at 1 year; P<0.0001; and sub-HR, 1.22 [1.15-1.30] at 3 years; P<0.0001). Device-related complications within 90 days did not differ between those with and without diabetes mellitus (odds ratio: 0.90 [0.77-1.06]; P=0.37). Interactions of age, sex, ischemic cardiomyopathy, renal failure, or QRS duration were not significant. In older patients with heart failure receiving cardiac resynchronization therapy with defibrillator, diabetes mellitus was independently associated with greater risks of death and rehospitalization, but similar risks of procedural complications.