We explored the relationship between abdominal vein thromboses, including portal vein thrombosis (PVT) and superior mesenteric vein thrombosis (SMVT), and thrombophilia. The frequency of thrombophilia, such as antithrombin (AT), protein C (PC), or protein S (PS) gene mutations, was examined in 21 patients with PVT, 6 patients with SMVT, and 6 patients with both PVT and SMVT. Low levels of AT, PC, or PS were frequently detected in patients with PVT or mesenteric vein thrombosis, and 4 mutations in the PS gene, 3 mutations in the PC gene, and 2 mutations in AT the gene were detected. Protein S Tokushima was detected in 3 of 4 patients with a PS gene mutation and was associated with 2 other PS gene mutations. The onset of PVT or SMVT was almost idiopathic in patients with congenital thrombophilia. Both PVT and SMVT were frequently caused by an AT, a PC, or a PS mutation, and the onset of these thromboses due to thrombophilia was frequently idiopathic.