Introduction: Linear lesion between the left inferior pulmonary vein orifice and mitral annulus, the so-called mitral isthmus, may improve the success of catheter ablation for atrial fibrillation. Gaps in the lesion line may facilitate left atrial flutter. The aim of the study was to determine the optimal location of the lesion line by serial measures of distances between pulmonary veins and mitral annulus using angio-computed tomography. Methods: Data from 46 ablation procedures of atrial fibrillation were analyzed (34 males, mean age 52 years). Left atrial diameters were measured: supero-inferior, latero-lateral, and antero-posterior. Three types of mitral isthmus lines were also measured: distance between right superior pulmonary vein – mitral annulus; right inferior pulmonary vein – mitral annulus and left inferior pulmonary vein – mitral annulus. Results: Angio-CT images were integrated in the Carto 3 system (Biosense Webster R) using image integration tool provided by the system. Th e supero-inferior diameter of left atrium was 44 mm, antero-posterior = 42,5 mm and latero-lateral = 59,8 mm. Due to position of left atrium in the mediastinum between vertebral column and aorta, enlargement of its diameter is made predominantly using the latero-lateral diameter. Th e shortest mitral isthmus was between LIPV and MA = 25 mm. Th e other 2 lines had a value of 29 mm for RIPV-MA and 52 mm for RSPV-MA. Left circumfl ex artery, coronary sinus were closest to LIPV-mitral annulus line. Conclusions: Angio-computed tomography provided detailed information regarding the anatomy of left atrium and distances between atrial structures. Th e line between LIPV –mitral annulus was the shortest among the three mitral isthmus lines. Th e circumfl ex artery sometimes very close to the endocardium, the presence of the coronary sinus with or without a myocardial sleeve around it, and local cooling by atrial arteries and veins may complicate the creation of a line of block in the mitral isthmus.