Background: Spontaneous conversion of atrial fibrillation (AF) often leads to left atrial (LA) stunning. Typically, LA contraction is regarded as LA function. However, reservoir function is governed by atrial compliance, which plays an important role in modulating LA function. We sought to evaluate which parameter is more representative index for LA function. Methods: A total 50 patients (56짹14 years) with recent onset (< 2 months)lone paroxysmal AF(PAF) were enrolled and compared with age-, gender-, and LV mass index-matched 35 controls (55 짹 12 years). All patients were in LV ejection fraction (EF)>50%, no evidence of ischemic heart disease or heart failure. LA function was assessed with the; a) LA filling volume and LA expansion index (EI) for indices of reservoir function, b) LA active emptying volume and LA active emptying fraction for indices of atrial contraction. For overall LA systolic function, fractional shortening, EF, kinetic energy, and ejection force were also calculated. Results: LA EF and fractional shortening were significantly decreased in PAF. However, LA ejection force, kinetic energy, and A-wave were comparable between two groups. The reservoir function (LAEI) was significantly decreased in PAF, but atrial contraction function was similar between two groups. Conclusions: In patients with recent onset lone PAF, overall LA function was decreased, especially in reservoir function, but preserved in atrial contraction. Therefore, reservoir function is more representative index for LA function than atrial contraction.