BACKGROUND: We performed electrical cardioversion for the patients with chormic nonvalvular atrial fibrillation under the transesophageal echocardiographic guidance after anticoagulation to evaluate the safety of this procedure and the effects of electrical cardioversion on the atrial function. METHODS: After anticoagulation therapy with coumadine for three weeks, we tried chemical cardioversion with amiodarone first. Failed cases were included in this study. Pre-cardioversion transesophageal echocardiographic parameters were measured after exclusion of thrombi. After sedation with intravenous midazolam, direct-current cardioversion was done with the transesophageal echocardiographic probe in situ. Immediately after sinus conversion, we measured echocardiographic parameters again. Spontaneous echo contrast(SEC), left atrial appendage flow velocity, pulmonary vein flow velocity and time-velocity-integral(TVI), transmitral flow velocity, TVI and deceleration time were measured. All patients were anticoagulated for at least 4 weeks after cardiovesion. RESULTS: The total number of patients was forty one(24 males, 17 females) with the mean age of 58 years(range : 39-70). Mean duration of atrial fibrillation was 65 months(range : 1-360). Hypertension(12), dilated cardiomyopathy(10), cerebrovascular accidents(6), ischemic heart disease(2) and chronic lung disease(1) were associated. There were no complications. SEC increased or newly appeared in 18(43.9%) patients after sinus conversion. The left atrial appendage emptying velocity decreased(32.8+/-17.4 vs. 22.1+/-11.4cm/sec, p=0.020) and systolic TVI of both upper pulmonic vein increased significantly after sinus conversion. In two cases, early systolic forward flow(S1) of pulmonic vein appeared after sinus conversion. Transmitral E velocity decreased(86.9+/-28.8 vs. 76.3+/-30.6cm/sec, p=0.006) and the deceleration time increased(164+/-49 vs. 206+/-53msec, p=0.000) after sinus conversion. Transmitral A velocity was still low(34.9+/-19.5cm/sec) and E/A ratio was high(2.6+/-1.4) immediately after sinus conversion. CONCLUSION: After appropriate anticoagulation therapy and exclusion of left atrium and left atrial appindage thrombi with TEE we could perform electrical cardioversion safety without complications. The changes in transesophageal echocardiographic parameters after sinus conversion revealed the appearance of atrial mechanical activity in concordance with electrical activity. But these findings suggested atrial stunning or electromechanical dissociation which necessitates extended anticoagulation therapy until the full recovery of atrial mechanical function.