The efficacy of amiodarone was assessed in 38 patients with atrial fibrillation resistant to quinidine and an effort made to identify factors correlated with amiodarone response. The study group included 29 patients with and 9 without organic heart disease and either persistent (n = 11) or paroxysmal (n = 27) atrial fibrillation. All patients were treated with amiodarone and followed up in a research clinic. Efficacy was classified as excellent (no recurrent symptomatic atrial fibrillation) in 15 (55%) of 27 patients with paroxysmal and 5 (45%) of 11 patients with persistent atrial fibrillation. Efficacy was poor (no effect on atrial fibrillation) in 5 (19%) of 27 patients with paroxysmal and 6 (55%) of 11 patients with persistent atrial fibrillation. Efficacy was good (amelioration but not total suppression) in 7 (26%) of 27 patients with paroxysmal atrial fibrillation. Efficacy was related to echocardiographic left atrial dimension, left ventricular ejection fraction and, in patients with persistent atrial fibrillation, the duration of the arrhythmia. During the follow-up period of 15 months (range 1 to 36), overall efficacy (considering response and toxicity) was 67% in the 27 patients with paroxysmal and 45% in the 11 patients with persistent atrial fibrillation. It is concluded that amiodarone offers an additional therapeutic alternative in quinidine-resistant atrial fibrillation and that certain clinical factors are correlated with amiodarone response.