Differentiation between typical and atypical atrial flutter solely based upon surface ECG pattern may be limited. However, successful ablation of atrial flutter depends on the exact identification of the responsible re-entrant circuit and its critical isthmus. Between August 2001 and June 2003, we performed conventional entrainment pacing within the cavotricuspid isthmus in 71 patients with sustained atrial flutter. In patients with positive entrainment we considered the arrhythmia as typical flutter and treated them with conventional ablation of the cavotricuspid isthmus. As a consequence of negative entrainment we performed 3D-electroanatomic activation mapping (CARTO trade mark ). Conventional ablation of the right atrial isthmus was successful in all patients (n = 54) with positive entrainment. We performed electroanatomic mapping in the remaining 17 patients (14 male; age 60.9 +/- 16 years) resulting in the identification of 6 cases with typical and 11 cases with atypical flutter. Therefore, entrainment pacing was able to predict the true presence of typical atrial flutter in 91.5%. Atypical flutter was right sided in 4 patients and left sided in 7 cases. Electrically silent ("low voltage") areas probably demonstrating atrial myopathy were identified in all cases with left sided and in 2 patients with right sided flutter. In these patients targets for ablation lines were located between silent areas and anatomic barriers (inferior pulmonary veins, mitral respectively tricuspid annulus, or vena cava inferior). In 1 patient, the investigation was stopped due to variable ECG pattern and atrial cycle lengths. In the remaining cases, ablation was acutely successful. One patient, after surgical closure of a ventricular septal defect, demonstrated a dual-loop intra-atrial reentry tachycardia dependent on two different isthmuses. This arrhythmia required ablation of those distinct isthmuses to be interrupted. After a mean follow-up of 8.8 +/- 3.4 months, there was one patient with a recurrence of left-sided atrial flutter. Another patient developed permanent atrial fibrillation shortly after the procedure. Mean duration time of the procedure was 235.6 +/- 56.4 min (right atrium: 196 +/- 17.3 min; left atrium: 267.2 +/- 59.5 min), and average fluoroscopy time was 21.8 +/- 11.7 min (right atrium: 9.5 +/- 6 min; left atrium: 28.9 +/- 7 min). There was no incidence of serious complications associated with these procedures. In conclusion, conventional pacing in the cavotricuspid isthmus combined with electroanatomic mapping was an effective method to differentiate between typical and atypical atrial flutter. Electroanatomic mapping was a powerful tool both for identification of different atrial re-entrant circuits including their critical isthmuses as well as for effective application of individual ablation line strategies.