Abstract Purpose: To determine the effects of medial (inwards) and lateral (outwards) rotation of the head on the transverse diameter of the internal jugular vein. Material: The original study sample included 26 patients. Duplex ultrasound was used to measure the transverse diameter of the internal jugular vein at a fixed point on either side of the neck. Measurements were taken with the head central (neutral position), rotated laterally and medially. Results: Using Student's t -test we found that lateral rotation of the head produced a reduction in the mean of the transverse diameters of the left and right ipsilateral vein from 6.9 mm to 5.4 mm ( p<0.03) and 5.9 mm to 5.0 mm ( p=0.2173) respectively. One patient excluded from the study because of previous neck surgery showed complete occlusion of the ipsilateral internal jugular vein on lateral rotation of the head. Conclusion: There is a possibility that patency of the vein could be compromised if the head is turned laterally. This situation may arise immediately after surgery in the ventilated and paralysed patient when the head may be unsupported. It could be of particular importance if the vein has been used as a recipient vein for free tissue transfer.