Left atrial myocardial extensions over the pulmonary veins (PVs), known as myocardial sleeves, are present in the physiological anatomy of most individuals. Although this structure has recently received clinical attention as a major origin of paroxysmal atrial fibrillation (AF), it has not been documented in surgical specimens. Here, we examine incidentally identified myocardial sleeve tissue in routinely processed lung resection specimens to determine its incidence and diagnostic implications. Among 694 lung resection specimens with evaluable PV margins, myocardial sleeve tissue was identified in 26 cases (3.7%). The tissue was located within the adventitia of the PVs, mostly in margin preparations, and existed outside the pericardium in the majority of cases. Carcinoma infiltration of the sleeves was evident in 6 cases. No heart injuries were observed, and no tumors invaded the heart. Preoperative electrocardiography showed sinus rhythm in all cases, whereas postoperative monitoring revealed sinus rhythm in all patients except one who showed AF and flutter. Myocardial sleeve tissue is an underrecognized incidental finding in lung resection specimens, and it is not indicative of heart injury. Cancer infiltration into this tissue indicates neither heart invasion nor, by itself, invasion into the pericardium. Although surgical transection of the myocardial sleeve did not evoke immediate arrhythmia in most cases, the overall influence of this procedure on the postsurgical risk of AF remains to be determined in further studies involving extensive rhythm assessment.