Purpose: Radiofrequency catheter ablation of the cavotricuspid isthmus (CTI) has been known as an effective therapy of typical atrial flutter (AFL) and atrial fibrillation (AF). AF and AFL often coexist. However, AF which was usually not presented was often occurred following successful ablation of CTI. The aim of this study was to investigate the predictor of concomitant AF following successful ablation of AFL. Methods: We enrolled 120 patients (59.1짹11.3 yrs, male 98 (81.7 %)) with typical AFL, who received successful ablation of the CTI. They were followed up at outpatient clinic (15.5짹17.7 months, range 3 to 105 months). The 12-lead ECG and Holter monitoring were used to confirm the diagnosis of recurrent AFL or AF. We assessed prior AF history, structural heart disease, left ventricular ejection fraction, left atrial diameter (LAD), left atrial volume index(LAVI) and AFL cycle length. Results: Among the 120 ablated patients, 14 (11.7 %) patients had recurrent AFL and 30 (25.0 %) had recurrent AF. Multivariate logistic regression analysis found that the independent predictors of recurrent AF were LAD and LAVI. The LAD of 4.5 cm and LAVI of 42.6 mL may allow for the differentiation between only AFL and AFL with concomitant AF with 72.7%, 77.3% of the sensitivity and 64.9%, 68.4% of the specificity, respectively. Conclusions: LAVI might be useful predictor for differentiation AFL with concomitant AF from only AFL. However, further larger study might be needed.