BACKGROUND The efficacy of radiofrequency (RF) ablation of an uncommon coronary sinus (CS)-dependent atrial flutter (AFL) was evaluated using conventional electrophysiological criteria in a highly selected subset of patients with typical and atypical AFL.METHODS Fourteen patients with atrial flutter (11 males, mean age 69±9 years) without previous right or left atrial RF ablation were included.Heart disease was present in eight patients.Baseline ECG suggested typical AFL in 12 patients and atypical AFL in two.Mean AFL cycle length was 324±64 ms at the time of RF ablation in the CS.Lateral right atrium activation was counterclockwise (CCW) in 13 patients and clockwise in one.CS activation was CCW in all Criteria for CS ablation included the presence of CS mid-diastolic fractionated atrial potentials (APs) associated with concealed entrainment with a postpacing interval within 20 ms.Success was defined as termination of AFL and subsequent noninducibility.RESULTS The initial target for ablation was the cavotricuspid isthmus (CTI) in 11 patients and the CS with further CTI ablation in three.AP duration at the CS target site was 122±33 ms, spanning 40±12% of the AFL cycle length.CS ablation site was located 1-4 cm from the CS ostium.Ablation was successful in all patients.Mean time to AFL termination during CS ablation was 39 ±52 s (<20 s in eight patients).No recurrence of ablated arrhythmia occurred during a follow-up of 18±8 months.CONCLUSIONS The CS musculature is a critical part of some AFL circuits in patients with typical and atypical AFL.AFL can be terminated in patients with CS or CTI/CS AFL reentrant circuits by targeting CS mid-diastolic fragmented APs.