PURPOSE: To evaluate the portal venous anatomy in the right lobe of the liver, focusing particularly on the location and size of the anterior and posterior segmental branches of the portal vein and the relationship of the right subdiaphragmatic peripheral portal vein to the right hepatic vein. MATERIALS AND METHODS: From June 1995 to December 1995, 100 spiral CT scan which showed no abnormal findings in the hepatic area were retrospectively analysed. Portal dominant phase images were obtained after the administration of contrast media, with a delay of 60-65 seconds (100 - 120ml, 2-3ml/sec injection rate), slice thickness 10 mm and table speed 10mm/sec. On spiral CT scans, we assessed the location and size of the right portal vein and its branches and also observed the relationship of this vein to the right hepatic vein. RESULTS: In all patients, the right portal trunk divided into anterior and posterior branches. The anterior segmental portal vein was located cephalad to the posterior segment in 81 cases (81%), at the same level in 17 (17%), and caudad in two (2%). Its diameter was greater (>2mm)than that of its posterior segment in 33 cases (33%), smaller in three (3%), and similar in 64 (64%). In 95 cases, the right anterior segmetal portal vein which was directed posteriorly, supplied the subdiaphragmatic portion of segment 7. CONCLUSION: In 81% of cases, the position of the anterior segmental portal vein cephalad, and in 64%of cases it was similar in size to the posterior portal vein. In almost all cases, the subdiaphragmatic portion of segment 7 was supplied by the portal vein from segment 8. Therefore, the right hepatic vein is not in all cases an adequate landmark for dividing Couinaud segments 7 and 8 in the subdiaphragmatic portion.