Background and Objectives: Normal mechanical atrial function is one factor for maintaining sinus rhythm(SR) in cardioverted atrial fibtrillation(AF). Atrial function immediate after cardioversion frequently diminished and method for evaluation of atrial functional reserve is not certain. This study was undertaken to evaluate atrial mechanical response to isoproterenol as a surrogate for atrial functional reserve responsible for maintaining SR in cardioverted AF patients.Methods:Ten electrically cardioverted AF patients were studied(M:F=7:3,58짹3yrs). Baseline transthoracic and transesophageal echocardiography(TTE,TEE) were done in all cases. Immediately after restoration of SR,left atrial appendage emptying and filling velocities(LAAEV,LAAFV) were measured by TEE after isoproterenol infusion with gradually incresing dose. TTE was done at 1week,1month,3months after cardioversion. Patients were divided into 2 groups(Group1/2=SR/AF recurred=7/3). Results:Baseline clinical and echocardiographic findings were similar between two groups except that the duration of AF was longer in Group2 than in Group1(11짹3vs 32짹3 months;p<0.05). In both groups,LAAEV and LAAFV decreased with termination of AF**. Isoproterenol increased LAAEV and LAAFV also in both groups**,but the increment of flow velocities in LAA at each isoproterenol dose was attenuated in Group2. Ejection fraction(EF) increased at 1month in Group1,but this change was not observed in Group2**. Mitral A velocity tended to increase at follow up in Group1. Conclusion:Immediately after cardioversion of AF, the improvement of LAA flow velocities by isoproterenol was more prominent in group with maintained SR than in group with recurred AF. It suggests that atrial functional reserve presented by increase of LAA flow velocities by isoproterenol may be a predictor of maintenance of SR after cardioversion. (**Data in table)