Four patients with unilateral mass lesions are presented in whom bilateral supratentorial subarachnoid pressures were continuously recorded. A significant pressure differential between the ipsi- and contralateral side was documented in each case. The possible relationship of this phenomenon to various factors involved in producing increased intracranial pressure, including cerebrospinal fluid flow dynamics, vascular reactivity, elasticity, and brain tissue pressure, are discussed. This study suggests that supratentorial subarachnoid pressure should be measured ipsilateral to the site of a focal mass lesion.