Backgrounds: Although lone paroxysmal atrial fibrillation (PAF) is clinically defined as an arrhythmia that occurs in the absence of structural heart disease or any other causal factors, there has been revealing evidence of abnormal atrial substrate. The purpose of this study is to assess whether patients with lone PAF have structural and functional abnormalities of the left atrium (LA). Methods and Results: A total of 20 patients with lone PAF (mean age: 56짹12 years, 75% men) and age-, sex- and body mass index-matched control subjects (n=20) were enrolled. An echocardiography was conducted more than 7 days after AF termination without any antiarrhythmic drugs. There were no significant differences in left ventricular (LV) cavity, LV mass index, LV ejection fraction (65.15짹6.08 % vs. 66.00짹3.08 %, p=0.581), LV diastolic dysfunction grade (0.70짹0.66 vs. 0.60짹0.50, p=0.336), and passive LA emptying fraction (LAEFpassive 22.65 짹9.63 % vs. 23.80짹8.38 % p=0.689) between groups. However, compared to controls, patients with PAF had significantly larger LA volume (LAVMax: 60.11짹21.01 ml vs. 47.13짹10.37 ml, p=0.02, LAVMDV: 46.63짹17.57 ml vs. 35.95짹8.63 ml, p=0.021, LAVmin: 37.33짹13.05 ml vs. 25.90짹6.63 ml, p=0.002) and lower total LAEF (LAEFtotal 37.60짹8.29 % vs. 45.10짹8.26 % p=0.007) and active LAEF (LAEFactive 18.90짹7.65 % vs. 27.30짹11.55 %, p=0.01). Conclusions: LAV and LAEF in patients with lone PAF had significantly larger and lower than in control group, despite of similar LV systolic and diastolic functions. These findings suggested that patients with lone PAF have occult structural and functional abnormalities of the LA.