The aim of head injury management is to prevent secondary insults to the damaged brain. Raised intracranial pressure and low cerebral perfusion pressure are two secondary insults which are important determinants of outcome following severe head injury (SHI). Traditionally ICP is measured in the right frontal region in an attempt to minimise the effects and complications of transducer placement. This assumes that the brain acts like a fluid and that ICP is transmitted equally throughout the intracranial space. Experimental studies suggest that this is not the case: expanding mass lesions are associated with the development of ICP gradients. Ten patients with SHI who had an unilateral mass lesion confirmed on CT were studied. All had bilateral placement of intraparenchymal Camino ICP transducers in the frontal regions. Data from both transducers were recorded every two minutes and stored electronically. The volume of the mass lesion was calculated from the CT scan. Significant and lasting ICP gradients between hemispheres were found in all patients with an acute subdural haematoma (greater than 10 mmHg for longer than 10 minutes). Such differences were not found in patients with intracerebral haematoma or contusions. We would advocate that ICP is recorded IPSILATERAL to the lesion in patients with SHI due to acute subdural haematoma.