Context: Although left atrial radiofrequency ablation (RFA) is increasingly used for the treatment of chronic atrial fibrillation during mitral valve surgery, its efficacy to restore sinus rhythm and any resulting benefits have not been examined in the context of an adequately powered randomized trial. Objective: To determine whether intraoperative RFA of the left atrium increases the long-term restoration of sinus rhythm and improves exercise capacity. Design, Setting, and Patients: Randomized, double-blind trial performed in a single UK tertiary referral center with enrollment between December 2001 and November 2003. A total of 101 patients referred for mitral valve surgery with at least 6 months' history of uninterrupted atrial fibrillation were assessed for eligibility; 97 were enrolled. Patients were followed up for 12 months. Intervention: Patients were randomly assigned to undergo mitral valve surgery and RFA of the left atrium (n=49) or mitral valve surgery alone (controls; n=48). Main Outcome Measures: The primary outcome measure was presence of sinus rhythm at 12 months; secondary measures were patient functional status and exercise capacity (assessed by shuttle-walk test), left atrial contractility, and left atrial and left ventricular dimension and function and plasma levels of B-type natriuretic peptide. Results: At 12 months, sinus rhythm was present in 20 (44.4%) of 45 RFA patients and in 2 (4.5%) of 44 controls (rate ratio, 9.8; 95% CI, 2.4-86.3; P<.001). Restoration of sinus rhythm in the RFA group was accompanied by a greater improvement in mean (SD) shuttle-walk distance compared with controls (+94 [102] m vs +48 [82] m; P=.003) and a greater reduction in the plasma level of B-type natriuretic peptide (-104 [87] fmol/mL vs -51 [82] fmol/mL; P=.03). Patients randomized to receive RFA had similar rates of postoperative complications and deaths as control patients. Conclusions: Radiofrequency ablation of the left atrium during mitral valve surgery for continuous atrial fibrillation significantly increases the rate of sinus rhythm restoration 1 year postoperatively, improving patient exercise capacity. On the basis of its efficacy and safety, routine use of RFA of the left atrium during mitral valve surgery may be justified. Trial Registration: ClinicalTrials.gov Identifier: NCT00238706. ©2005 American Medical Association. All rights reserved.