This double-blind, placebo-controlled study is designed to evaluate the efficacy and safety of azimilide compared with placebo in maintaining sinus rhythm in patients who require cardioversion (electric shock to correct heart rhythm) to reduce atrial fibrillation. Once this phase of the study is completed, a second phase with a different study design will be conducted. The second phase is an open-label follow-up phase to the study. This follow-up phase will continue to evaluate the long-term efficacy and safety of a daily oral dose of azimilide in patients who complete the double-blind, placebo-controlled phase of this study.