Interface pressures beneath six types of collar in volunteers were measured. Under certain types of collar mean interface pressures of more than 10 mmHg were obtained, with several individual readings around 30 mmHg. By causing jugular venous obstruction a similar rise in intracranial pressure may be produced inadvertently. The mechanisms of action for this are discussed. Those collars with the least tendency to produce jugular venous obstruction were of the moulded variety which extend support to the occiput, mandible and shoulder girdle. The authors therefore recommend this type of cervical splintage device where injury of the cervical spine is suspected. This paper emphasizes the potential danger to head-injured patients when the other devices are applied too tightly, producing a 'venous tourniquet' effect.