Background Although radiofrequency catheter ablation of the atrioventricular (AV) node is an established treatment for atrial fibrillation (AF) with uncontrolled ventricular response, factors that predict clinical outcome in patients with associated congestive heart failure (CHF) are unknown. Methods and Results AV node ablation and permanent pacemaker implantation was performed in 44 consecutive patients (mean age 71 ± 10 years) with CHF and AF associated with uncontrolled ventricular response. Immediately before ablation, mean left ventricular ejection fraction (EF) measured by 2-dimensional echocardiogram was 34.6% ± 9.8%, mean exercise tolerance time was 2.6 ± 1.8 minutes, and mean quality of life score was 62.3 ± 19.7. Complete AV block was achieved in all 44 patients but was complicated by death in 1 patient from cardiogenic shock soon after ablation. By 1 month after ablation, EF increased to 43.8% ± 13.7% (P < .01), exercise tolerance time was 4.0 ± 2.5 minutes (P < .01), and mean quality of life score decreased to 35.6 ± 18.1 (P < .01). Improved cardiac performance (increase in EF ≥9% over baseline EF) was detected in 20 (45%) of the patients. During a mean follow-up of 17 ± 9 months, 5 patients died suddenly of presumed ventricular tachyarrhythmia and 4 others died of progressive CHF. Multivariate Cox survival analysis identified baseline EF ≤30%, presence of significant mitral regurgitation (>2+) before ablation, and failure to exhibit improved cardiac performance by 1 month after ablation as the only independent predictors of death. Conclusions Baseline variables and failure of EF to improve soon after AV node ablation identifies patients with CHF and AF who have a high mortality rate. Adjunctive therapy to reduce sudden death and progressive heart failure should be evaluated in this subgroup. (Am Heart J 1998;136:647-57.)