We defined the subsegmental divisions and the ramification patterns of the portal vein in the right and caudate lobes using 25 human liver casts. The ramifications of the portal vein and the subdivisions of the liver were classified based on the major portal veins with the largest diameter and those having a diameter of not less than two thirds of the largest vein in each subsegment. The following results were obtained. (l)The portal trunk showed three ramification patterns and the basic pattern was bifurcation (80%). (2) The anterior portal vein first ramified into several anterior-inferior portal veins (P5) and ran toward the superior direction to bifurcate into 2 major portal veins in the anterior-superior subsegment (Sg). (3)There were three types of ramification patterns of the portal veins in S8: bifurcation (84%), trifurcation and one-pedicle type. (4)There were also three branching types of the largest vein (P5-max) in P5: ramification from the anterior portal vein, Pg-anterior vein supplying the anterior region of S8 and Pg-posterior vein supplying the posterior region of S8. (5)The posterior portal vein showed two ramification patterns of the bifurcation (36%) and nonbifurcation type. (6) The major portal veins in the caudate subsegment ramified at various sites such as the portal trunk, left, right and/or other portal veins.