BACKGROUND AND OBJECTIVES: The sinus venosus (SV) is not a well known source of atrial tachycardia (AT), but it can harbor AT during catheter ablation of atrial fibrillation (AF). SUBJECTS AND METHODS: A total of 1223 patients who underwent catheter ablation for AF were reviewed. Electrophysiological and electrocardiographic characteristics and outcomes after catheter ablation of AT originating from the SV were investigated. RESULTS: Ten patients (0.82%) demonstrated AT from the SV (7 males, 53.9+/-16.0 years, 6 persistent) during ablation of AF. The mean cycle length was 281+/-73 ms. After pulmonary vein isolation and left atrial ablation, AF converted to AT from the SV during right atrial ablation in 2 patients, by rapid atrial pacing after AF termination in 7 patients, and during isoproterenol infusion in 1 patient. Positive P-waves in inferior leads were shown in most patients (90%). The activation sequence of AT was from proximal to distal in the superior vena cava and high to low in the right atrium, which was similar to that of AT from crista terminalis. Fragmented double potentials were recorded during sinus, and a second discrete potential preceded the onset of P wave by 80+/-37 ms during AT. Using 4.4+/-2.7 radiofrequency focal applications, ATs were terminated and became no longer inducible in all. After ablation procedure, two patients showed transient right phrenic nerve palsy. After 19.9+/-14.8 months, all but 1 patient were free of atrial tachyarrhythmia without complications. CONCLUSION: The AT which develops during AF ablation is rarely originated from SV, and its electrophysiologic characteristics may be helpful in guiding effective focal ablation.