A 66-year-old man with cor triatriatum sinister underwent pulmonary vein (PV) isolation (PVI) for atrial fibrillation (AF) twice because of AF recurrence. Different transseptal approaches into the anterior chamber receiving the left atrial appendage and posterior chamber receiving the PVs, were achieved in two sessions. PVI of the left PVs and right superior PV was challenging via the anterior chamber, whereas PVI of the right inferior PV was challenging via the posterior chamber because of the disturbance of the membrane. Therefore, an intentional transseptal catheterization into the more appropriate chamber may be necessary for PVI in a cor triatriatum sinister. © 2008 Springer Science+Business Media, LLC.