Background - Left atrial volume (LAV) is an important marker of heart failure (HF) severity. We hypothesized that LAV independently correlates with clinical outcomes in patients who receive cardiac resynchronization therapy with a defibrillator (CRT-D) and can be used for improved risk assessment in this population. Methods and Results - The benefit of CRT-D versus defibrillator-only therapy in reducing the risk of HF or death was assessed by LAV (dichotomized at the upper quartile >52 mL/m2) among 1785 patients enrolled in the Multicenter Automatic Defibrillator Implantation Trial with Cardiac Resynchronization Therapy (MADIT-CRT) study. Landmark analysis was used to evaluate the relationship between LAV response to CRT-D and subsequent clinical outcomes. Multivariable analysis showed that patients with a higher baseline LAV experienced 69% (P<0.001) and 59% (P=0.02) increased hazard for HF or death and for all-cause mortality, respectively, independently of baseline left ventricular volume. CRT-D was associated with a significant reduction in LAV compared with defibrillator-only therapy (-28% versus -10%, respectively; P<0.001). Landmark analysis showed that after CRT-D implantation each 1% reduction in LAV was independently associated with a corresponding 4% reduction in the hazard of subsequent HF or death (P<0.001). The assessment of LAV change after CRT implantation improved prediction of clinical response to the device compared with assessment of the corresponding changes in left ventricular volume. Conclusions - LAV is an independent correlate of clinical outcomes in mildly symptomatic HF patients treated with CRT-D. CRT exerts pronounced reverse remodeling effects on the left atrium that independently correlate with improved clinical outcomes after device implantation. © 2013 American Heart Association, Inc.