Objective: This study was prospectively designed to evaluate the relationship between the mitral A wave acceleration slope (AWA-slope) and the left atrial ejection fraction (LA-EF) after the restoration of sinus rhythm in patients with atrial fibrillation (AF), and also to evaluate the change in the AWA-slope between the sequent second day and first month. Methods: Twenty-nine patients (16 female, mean age 56±13 years) with unknown cause of AF (except for age) converted to sinus rhythm within the first 48 hours were included into the study. A transthoracic echocardiography was performed in all patients on the second day and the first month after restoration of the sinus rhythm. The paired Student’s t test was used in comparisons of the continuous variables. The simple and multiple correlations of the LA-EF were evaluated by the simple and multiple linear regression analyses, respectively. Results: Left atrial ejection fraction (42±17 vs 51±19%, p=0.03) and AWA-slope (950±337 vs 1087±351 cm/sec2, p=0.021) obtained after the first month were greater than on the second day. A significant correlation between LA-EF and AWA-slope was observed both on the second day (r=0.76, p<0.001) and at the end of the first month (r=0.71, p<0.001). In addition, there were the correlations between LA-EF and mitral A wave peak velocity (r=0.42, p=0.025) or mitral E/A ratio (r=-0.39, p=0.040) at the end of the first month. On multiple linear regression analysis, only AWA-slope was found to be related to LA-EF (y=9.35+0.04 (AWA-slope), the overall R2=0.51, beta =0.71; 95% CI 0.02-0.05; p<0.001). Conclusion: Mitral A wave acceleration slope is a simple, reliable and non-invasive method that can be used to evaluate left atrial contractile function. It can also be used in monitoring of atrial stunning period in patients with sinus rhythm converted from AF.