Intraluminal tumor thrombus in the portal vein (PV) system originating from gastrointestinal (GI) tract cancer is a rare condition. There are two types of such thrombi, one arising indirectly from metastatic liver cancer and the other directly from the primary lesion. We report here three patients with the direct type and two with the indirect type; i.e., a total of five patients with gastric or large intestinal cancer with PV tumor thrombus. In all patients, the primary lesion was surgically resected; in two patients, the tumor thrombus was easily extirpated by direct opening of the PV. It is noteworthy that a patient whose tumor thrombus could not be treated died of cancer with liver failure, caused by expansive growth of the PV tumor thrombus, 4 months after the finding of the PV thrombus. Because PV tumor thrombus may, possibly, determine the patient's length of survival, in addition to causing cancer progression, surgical thrombectomy, combined with resection of the primary cancer and metastatic liver cancer, should be considered for prolongation of survival, if all macroscopic lesions can be controlled and if the tumor thrombus is a synchronous and recent one.