The subarachnoid bolt has been used extensively to monitor intracranial pressure in a variety of conditions. We have had two patients who had subarachnoid bolts in place that were thought to be functional in whom evidence of increased pressure was present. In one case, the patient had Reye's syndrome with seizure activity, decerebrate posturing, fixed and dilated pupils, and cardiac arrest. Although autopsy revealed evidence of cerebellar and uncal herniation, verifying the presence of pressure cones, the subarachnoid bolt pressure was never elevated and had a good wave form. In the other case, the patient developed an epidural hematoma postoperatively while a subarachnoid bolt was in place. The subarachnoid bolt was measuring pressures of less than 15 mm Hg with a good wave form despite clinical and, subsequently, computed tomographic evidence of an evolving mass. The use of a pressure monitor such as the subarachnoid bolt should not replace frequent clinical assessment of the patient.