Objective: Trendelenburg positioning is a common approach used during internal jugular vein (IJV) cannulation. No evidence indicates that Trendelenburg positioning significantly increases the crosssectional area (CSA) of the IJV in obese patients. The primary aim of this study was to determine the effectiveness of Trendelenburg positioning on the CSA of the right internal jugular vein assessed with ultrasound measurement in obese patients. Methods: Forty American Society of Anesthesiologists II patients with body mass index ≥30 kg/m² undergoing various elective surgeries under general endotracheal anesthesia were enrolled. Ultrasound images of the right IJV were obtained in a transverse orientation at the cricoid level. We measured the CSA of the right IJV two different conditions in a sealed envelope were applied in random order: State 0, table flat (no tilt), with the patients in the supine position, and State T, in which the operating table was tilted 20° to the Trendelenburg position. Results: The change in the CSA of the IJV from the supine to the Trendelenburg position (1.80 cm² vs 2.08cm²) was not significantly different. The CSA was paradoxically decreased in 10 of 36 patients when the position changed from State 0 to State T. Conclusions: Trendelenburg positioning does not significantly increase the mean CSA of the right IJV in obese patients. In fact, in some patients, this position decreases the CSA. The use of the Trendelenburg position for IJV cannulation in obese patients can no longer be supported.