To determine the effect of Trendelenburg position on the diameter or cross-section area of the internal jugular vein (IJV) a systematic review and metanalysis was performed. Studies that evaluated the cross-sectional area (CSA) and anteroposterior (AP) diameter of the right internal jugular vein (RIJV) with ultrasonography in supine and any degree of head-down tilt (Trendelenburg position) were analyzed. A total of 22 articles (613 study subjects) were included. A >5° Trendelenburg position statistically increases RIJV CSA and AP diameter. Further inclination from 10° does not statistically benefit IJV size. This position should be recommended for CVC placement, when patient conditions allow it, and US-guided cannulation is not available.