Left upper lobectomy may be followed by complications such as thrombus formation in a stump of the left superior pulmonary vein (LSPV), which may cause systemic embolization. We have encountered four such cases, which account for 3.4% of all left upper lobectomies performed at our institution. Right renal infarction was observed in one of these four cases; the remaining cases were asymptomatic, with the thrombus incidentally detected by enhanced computed tomography (CT). The postoperative duration for the detection of the thrombus varied from 4 days to 24 months. Even in a case in which the superior pulmonary vein (PV) was divided by posterolateral thoracotomy, CT showed that the stump was long enough intrapericardially for thrombus formation. Anticoagulant therapy was administered in all the cases, resulting in dissipation of the thrombus. Therefore, when a thrombus is detected in a pulmonary stump, an anticoagulant should be administered.