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Introduction Left atrial perimitral flutter is a common late complication of atrial fibrillation ablation. Repeat ablation with a posterolateral mitral isthmus line to eliminate the flutter circuit has proven technically challenging, leading some to opt for anteroseptal line ablation as an alternative. A significant issue with the latter approach is the emergence of a biatrial flutter using the right atrial septum via epicardial connections to bypass the line of block and allow continued flutter propagation (image 1). In this case series we present our experience of biatrial flutter in patients with previous anteroseptal left atrial ablation.Methods We reviewed all cases of biatrial flutter treated at our institution from January 2017 to July 2024.Results Four cases of biatrial flutter were identified. Clinical and procedural characteristics are listed in table 1. All patients were male and had undergone prior pulmonary vein isolation and anteroseptal line. Ablation at sites of interatrial conduction successfully terminated biatrial flutter in 3 out of 4 cases. The remaining case had an unsuccessful ablation and was cardioverted to sinus rhythm.Conclusion Biatrial flutter is a potential complication following anteroseptal line formation for perimitral flutter, necessitating biatrial mapping for diagnosis. Ablation at interatrial connection sites where earliest activation is identified is a potentially effective treatment strategy.Abstract 60 Figure 1Abstract 60 Table 1 Case Age Sex Ablation prior to biatrial flutter Interatrial connections involved Ablation site Outcome 1 58 Male PVI, LAPWI, ASL BB, CS Superior RA septum, cavoatrial junction Successful 2 69 Male PVI, LARL, LAPWI, ASL, CTI, PLL BB (posterosuperior branch), CS High posterior RA Successful 3 75 Male CTI, ASL BB, CS Posteroseptal SVC Successful 4 70 Male PVI, ASL BB, CS Superior LA (close to BB insertion), PLL Unsuccessful (cardioverted) PVI = pulmonary vein isolation, LAPWI = left atrial posterior wall isolation, ASL = anteroseptal line, LARL = left atrial roof line, CTI = cavotricuspid isthmus, PLL = posterolateral line, BB = Bachmann’s bundle, CS = coronary sinus, RA = right atrium, SVC = superior vena cava