Objectives: Prior research has demonstrated that rigid cervical collars cause an increase in intracranial pressure (ICP). The mechanism for this effect is unclear and one proposed mechanism involves obstruction of venous outflow in the neck. Ultrasound (US) allows assessment of internal jugular vein dimensions and may yield information regarding the mechanism for the increase in ICP seen with rigid collar application.Methods: Forty-two healthy volunteers underwent US examination of the internal jugular vein before and after cervical collar application. Internal jugular vein cross-sectional areas were compared with and without the cervical collar in place. Results:The cross-sectional area of the internal jugular vein increased significantly (p < 0.0001) after application of the cervical collar. The mean percentage increase in cross-sectional area was 37% (95% confidence interval [CI] = 20% to 53%).Conclusions: Internal jugular vein cross-sectional area increases after application of a rigid cervical collar. This supports the hypothesis that venous obstruction in the neck may contribute to the increase in ICP seen after rigid collar application.