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Background: During high sacrectomies and lateral pelvic compartment exenterations, isolating the external and internal iliac veins within the presacral area is crucial to avoid inadvertent injury and severe hemorrhage. Anatomical variations of external iliac vein tributaries have not been previously described, whereas multiple classifications of internal iliac vein tributaries exist., Objective: We sought to clarify the iliac venous system anatomy using soft-embalmed cadavers., Design: This is a descriptive study., Settings: This study was conducted in Chulalongkorn University, Thailand., Patients: We examined 40 iliac venous systems from 20 human cadavers (10 males, 10 females)., Interventions: Blue resin dye infused into the inferior vena cava highlighted the iliac venous system, which was meticulously dissected and traced to their draining organs., Main Outcome Measures: Iliac vein tributaries and their valvular system were documented and analyzed., Results: The external iliac vein classically receives 2 tributaries (inferior epigastric and deep circumflex iliac) near the inguinal ligament. However, external iliac vein tributaries in the presacral area were found in 20 venous systems among 15 cadavers (75%). The mean diameter of each tributary was 4.0 ± 0.35 mm, with 72% arising laterally. We propose a simplified classification for internal iliac vein variations: pattern 1 in 12 cadavers (60%) where a single internal iliac vein joins a single external iliac vein to drain into the common iliac vein; pattern 2 in 7 cadavers (35%) where the internal iliac vein is duplicated; and pattern 3 in 1 cadaver (5%) where bilateral internal iliac veins drain into a common trunk before joining the common iliac vein bifurcation., Limitations: This study is limited by the number of cadavers included., Conclusions: A comprehensive understanding of previously unreported highly prevalent external iliac vein tributaries in the presacral region is vital during complex pelvic surgery. A simplified classification of internal iliac vein variations is proposed. See Video Abstract at http://links.lww.com/DCR/A900.