Subarachnoid pressure was measured by bifrontal catheterization to obtain concurrent comparative intracranial pressures (ICPs). These ICPs were compared in 15 head-injured patients who had differences in the pathological processes affecting the right and left hemispheres. ICPs in these patients ranged from -3 to 130 mm Hg, but no difference was found in the concurrent comparative ICPs. These findings suggest that the supratentorial space can generally be considered as one compartment regardless of the differences in types of intracranial lesions. (Neurosurgery 21:688-692, 1985)