Two cases of successful radiofrequency catheter ablation of adult-onset atrial tachycardia originating from the left atrium adjacent to the mitral annulus are presented. Endocardial catheter activation mapping performed by retrograde or atrial transseptal approach revealed presystolic activation at the successful ablation site in both patients, and fractionation during sinus rhythm and tachycardia in one. The 12-lead electrocardiographic P wave appearance was suggestive of a left atrial tachycardia origin in both cases.