Background: Randomized trials demonstrating the efficacy of implantable cardioverter defibrillator (ICD) therapy for primary prevention of sudden cardiac death in patients with systolic heart failure (HF) predominantly used single chamber devices. However, in clinical practice, patients often receive dual chamber ICDs even in the absence of indications for pacing. The long-term safety of dual chamber devices relative to single chamber devices is uncertain. Methods: We identified all Medicare patients in the National Cardiovascular Data Registry's (NCDR®) ICD Registry™ from 2006-2009 who received an ICD for primary prevention that could be matched to CMS claims data. Patients were excluded if they had an EF >35%, received a bi-ventricular device or had a documented indication for pacing. Adjusted risks of complications, mortality, all cause readmission and HF readmission were estimated with propensity-score matching based on demographic, clinical and diagnostic information. Results: Among 32,034 eligible patients, 38% (n=12,246) received single chamber device and 62% (n=19,788) received a dual chamber device. Unadjusted rates of complications and 6-month all-cause readmission were lower among patients who received a single chamber device (3.5% vs. 4.8%; p <0.001, and 31.7% vs. 33.2%; p=0.004). According to analysis of matched pairs (12,133 single and 12,133 dual), rates of complications were lower among patients who received a single chamber device (3.5% vs.4.6%; p<0.001), but rates of all other outcomes did not differ significantly between the two groups. (Table) Conclusions: Dual chamber ICDs are associated with higher rates of complications. No significant differences between single and dual chamber ICDs were observed for death, all-cause readmission or HF readmission. Future studies should evaluate the potential benefit of dual chamber devices in reducing inappropriate shocks. In Medicare patients undergoing primary prevention ICD placement without clear indications for pacing, the decision to implant a dual chamber device should be considered carefully given the higher risk of complications with this strategy.