OBJECTIVES: To evaluate the serial changes in left atrial function we performed electrical cardioversion of chronic nonvalvular atrial fibrillation under the monitoring of transesophageal echocardiography(TEE) and followed the success cases with TEE. METHODS: All patients were anticoagulated from three weeks prior to and till four weeks after cardioversion. After exclusion of thrombi, direct-current cardioversion was done with the probe in situ. Immediately after comersion, TEE repeated. Follow-up TEE was done in cases whose rhythm were sinus over three months. Spontaneous echo rontrast(SEC): appendage emptying velocity, filling velocity, diameter, area, emptying fraction ', pulmonary vein systolic(S) velocity, diastolic(D) velocity, atrial reversal(AR) velocity, systolic time-velocity-integral(S - TVI), diastolic- TVI(D-TVI), S/D ratio, S-TVI/D-TVI ratio, AR duration : transmitral E velocity, A velocity, E- TVI, A- TVI, E/A ratio, A- TVI/E TVI ratio and deceleration time(DT) were measured. RESULTS: The number of patients was thirty-one(18 males) with the mean age of 58 years (range :39-69). Mean duration of AF was 56 months(range : 5-360). Mean follow-up duration was 8 months(range: 3-13). Hypertension(10), dilated cardiomyopathy(8), ischemic heart disease(2) and chronic lung disease(1) were associated. There were no complications, The SEC increased immediately after conversion and decreased at follow-up. The appendage emptpng velocity decreased after conversion and increased at follow-up significantly. The appendage emptpng fraction showed increasing tendency, but that was not statistically significant. The pulmonary vein S/D ratio and S - TVI/D - TVI ratio were increasing gradually after conversion and follow-up. The AR duration increased at follow-up. The transmitral E velocity was gradually decreasing after conversion and at follow-up. The E/A ratio decreased, the E-TVI decreased and the A-TVI/E-TVI ratio increased. The DT increased after conversion, but not changed significantly at follow-up. CONCLUSIONS: The immediate changes after conversion suggested "atrial stunning" or "electromechanical dissociation". The follow-up TEE suggested that left atrial function was recovering with maintenance of sinus rhythm.