BACKGROUND: Atrial fibrillation is a common arrhythmia in patients with left ventricular dysfunction associated with increased morbidity and mortality. The present study investigated the potential of dofetilide to restore and maintain sinus rhythm in patients with left ventricular dysfunction, which might reduce mortality and hospitalizations. METHODS AND RESULTS: In the Danish Investigations of Arrhythmia and Mortality ON Dofetilide (DIAMOND) studies, 506 patients were in atrial fibrillation (AF) or atrial flutter (AFl) at baseline. Over the course of study, cardioversion occurred in 148 (59%) dofetilide- and 86 (34%) placebo-treated patients. In these patients, the probability of maintaining sinus rhythm for 1 year was 79% with dofetilide versus 42% with placebo ( P < 0.001). Dofetilide had no effect on all-cause mortality, but restoration and maintenance of sinusrhythm (independent of study treatment) was associated with a significant reduction in mortality (risk ratio [RR], 0.44; 95% CI, 0.30 to 0.64; P < 0.0001). In addition, dofetilide therapy was associated with a significantly lower risk ratio versus placebo for either all-cause (RR, 0.70; 95% CI, 0.56 to 0.89; P < or = 0.005) or congestive heart failure (RR, 0.69; 95% CI, 0.51 to 0.93; P < or = 0.02) rehospitalization. CONCLUSIONS: Dofetilide is safe and increases the probability of obtaining and maintaining sinus rhythm in patients with structural heart disease. The present study suggests that restoration of sinus rhythm--on placebo or dofetilide--is associated with improved survival.