A 64-year-old man with complete heart block, status post-Medtronic dual chamber pacemaker insertion, failed ablation for atrial tachycardia at an outside institution. Despite persistent palpitations and known unsuccessful ablation, pacemaker interrogation revealed no evidence of atrial arrhythmias. At electrophysiology study, burst pacing from the high right atrium and distal coronary sinus at 370 ms revealed bidirectional 2:1 interatrial conduction block. Left atrial burst pacing at 260 ms induced an atrial tachycardia (cycle length 340 ms) with 2:1 left to right atrial block and right atrial activation at 680 ms. The tachycardia was localized to the lateral left atrial roof. A series of ablation lesions from left to right superior pulmonary vein terminated the tachycardia. Left to right interatrial conduction block is a mechanism for underdetection of atrial arrhythmias with implantable devices not previously described. As the extent of atrial ablation increases, the incidence of this mechanism of underdetection may increase. Though devices are often considered ideal for atrial arrhythmia detection and are used in multiple trials, detection failures can occur despite appropriate device function. This case underscores the need for electrocardiographic monitoring in addition to device-based electrogram monitoring.