Cardiac resynchronization therapy (CRT) improves morbidity and mortality in patients with heart failure with QRS>120 ms, yet most patients studied in clinical trials manifested baseline left branch bundle block (LBBB). It is unclear whether benefits of CRT extend to patients with right branch bundle block (RBBB) or a paced QRS at baseline despite QRS>120 ms. Orthotopic heart transplantation- and ventricular assist device-free survival, symptomatic response, and echocardiographic response were evaluated in the 636 patients who underwent CRT at our institution from 2000 to 2007 in whom the baseline electrocardiogram showed LBBB (n=412; 65%), paced QRS (n=162; 26%), or RBBB (n=62; 10%). Mortality was assessed using the Social Security Death Index, and the medical record was analyzed for clinical data. A decrease in New York Heart Association class>or=0.5 after >or=6 months of CRT defined symptomatic response. Echocardiographic evidence of improved left ventricular function and reverse remodeling was evaluated after >or=6 months of CRT. Survival free from orthotopic heart transplantation and ventricular assist device placement was best in patients with LBBB and worst in those with RBBB, whereas patients with paced QRS had an intermediate prognosis (p=0.003). This finding remained significant after controlling for baseline differences among the 3 groups. Symptomatic response was observed most often in patients with LBBB (60%), occurred least often in patients with RBBB (14%), and was intermediate in patients with paced QRS (46%; p<0.001). Echocardiographic improvement showed a similar stepwise trend. In conclusion, patients with RBBB undergoing CRT had low rates of symptomatic and echocardiographic response, and their survival free from orthotopic heart transplantation or ventricular assist device placement was significantly worse than in patients with LBBB. Patients with conventionally paced QRS experienced an intermediate response.