PURPOSE: Right internal jugular vein (RIJV) catheterisation is a common procedure in patients undergoing surgery. We aimed to compare diameters and the cross-sectional area (CSA) of the RIJV when the head is in a neutral or 30-degree rotated position during mechanical ventilation in various modes and spontaneous ventilation. METHODS: Thirty patients undergoing surgery were included in the study. In each patient, still ultrasound images of the vena jugularis interna were taken with the head in a neutral position and with the neck rotated 30 degrees to the left, first under spontaneous ventilation (group S), then after the induction of anaesthesia under volume-controlled ventilation (group V) and under pressure-controlled ventilation (group P). The six still images were evaluated in terms of transverse and anteroposterior diameters and CSA. RESULTS: Diameters in the neutral and lateral positions in group S were significantly smaller than in group P and group V (neutral transverse p = 0.01, anteroposterior p = 0.041, rotated transverse p = 0.01, anteroposterior p = 0.03). The CSAs of the RIJV in the neutral and lateral positions were significantly larger in group P and group V than Group S (lateral CSA p = 0.001, neutral CSA p = 0.002). CSA increased significantly only in group P when the head was rotated 30 degrees laterally (p = 0.002). CONCLUSION: We conclude that both pressure-controlled and volume-controlled mechanical ventilation have similar effects on the CSA and diameters of the RIJV. Positioning of the head with a 30-degree rotation laterally has different effects on CSA depending on the ventilation mode used. A neutral position should be preferred with spontaneous ventilation whereas 30 degree rotation should be preferred in patients under pressure-controlled and volume-controlled ventilation.