Left ventricular (LV) function was studied in 30 patients with lone atrial fibrillation (AF) (paroxysmal [n = 27] and persistent [n = 3]) before and after ablation of atrioventricular conduction. In all patients, drug treatment did not control ventricular rate during AF or prevent recurrences of the arrhythmia, or both. LV ejection fraction, and LV end-systolic and end-diastolic, and left atrial dimensions were measured by echocardiography before (mean 7 +/- 10 months, range < 1 to 37) and after (14 +/- 20 months, < 1 to 77) ablation. Before ablation, LV ejection fraction was < or = 50% in 12 patients (group I) and > 50% in 18 (group II). After ablation, LV ejection fraction increased significantly in group I from 43 +/- 8% to 54 +/- 7% (p < 0.0001). There were also significant decreases in LV-end systolic and end-diastolic, and left atrial dimensions. No changes in these parameters were observed in group II. Groups I and II had a significant difference in the duration of AF (group I: mean 11 years, range 8 to 28; and group II: 5 years, 2 to 14) (p < 0.05). No difference was present in age, sex, New York Heart Association functional class for dyspnea, or type of ablation procedure. Thus, some patients with lone AF may show deterioration of LV function, which appears to be related to the duration of the arrhythmia; in these cases, LV function may improve significantly after ventricular rate control is accomplished by ablation of atrioventricular conduction.