Spinal immobilization and the application of a rigid collar to protect the neck forms an integral part of care of the injured. The very nature of collar design predisposes to vascular obstruction of blood draining from the brain and theoretically may raise intracranial pressure (ICP). We analysed this effect prospectively in a series of injured patients using the Stifneck rigid collar, the most popular collar used in the UK. Comparison of the ICP before, during and after collar application showed a significant rise (P < 0.001), a mean rise in ICP of 4.5 mmHg, with a standard deviation of 4.1 mmHg. Insignificant changes in mean arterial pressure suggested that this effect is a response to distortion of venous drainage rather than cutaneous stimulation alone. Since head-injured patients with lowered level of consciousness form a key group who require cervical spinal immobilization it is essential that secondary insults producing raised ICP are minimized. Alternative forms of cervical spinal immobilization should be considered if collars impede venous drainage through the neck.