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The anti-arrhythmic drug amiodarone may be effective in maintaining a normal heartbeat in patients with atrial fibrillation who do not respond to other anti-arrhythmic drugs. Eighty-nine patients with persistent atrial fibrillation took 600 milligrams (mg) of amiodarone for four weeks. Those who did not convert to a normal rhythm were defibrillated, and switched to approximately 200 mg of amiodarone a day. Fifteen patients converted to a normal rhythm on the drug alone and 74 had to be defibrillated. Normal rhythm was restored in 90% of the patients. Only three patients had a severe reaction to the drug and none developed a secondary arrhythmia. One patient died of end-stage heart failure, but this was not attributed to the drug. More than half the patients still had a normal rhythm three years later., Objective.--To study efficacy and safety of low-dose amiodarone for maintenance of sinus rhythm after electrical cardioversion of atrial fibrillation or flutter. Design.--Nonramdomized trial; mean duration of follow-up, 20.7 months. Setting.--Referral center; institutional practice; both hospitalized and ambulatory care. Patients.--Eighty-nine consecutive patients having chronic atrial fibrillation or flutter and eligible for cardioversion. Patients had failed previous treatment aimed at maintaining sinus rhythm. During follow-up one patient was withdrawn because of side effects; all patients were available for follow-up. Intervention.--Before cardioversion, patients received 600 mg of amiodarone daily during a 4-week loading period. After conversion, the daily maintenance dose was 204[+ or -]66 mg (mean[+ or -]SD). Main Outcome Measures.--Arrhythmia recurrence and adverse effects causing drug discontinuation. Results.--During loading, 15 patients (16%) converted, and after electrical cardioconversion, 90% of all patients had sinus rhythm. Actuarially, 53% of these patients were still in sinus rhythm after 3 years. In patients with compromised left ventricular function, 93% maintained sinus rhythm after 6 months. One patient died due to congestive heart failure. Intolerable side effects occurred in one patient. No proarrhythmia was observed. Logistic regression analysis revealed that amiodarone was ineffective in patients with mitral stenosis or chronic arrhythmia. Conclusions.--Low-dose amiodarone is effective for maintaining sinus rhythm in patients with difficult to treat chronic atrial fibrillation or flutter and is associated with a low incidence of serious side effects.