For a study, researchers sought to investigate cerebral perfusion pressure (CPP) at the time cerebral blood flow (CBF) ceased to flow following brain death (BD). They predicted that a critical closure pressure (CrCP) might be attained before the CPP fell to 0 mmHg. About 14 patients with rising intracranial pressure (ICP) leading to BD were included. The use of transcranial duplex (TCD) ultrasonography was employed to investigate CBF. TCD was repeated until waveforms indicated a decrease of CBF, beginning with a CPP of 30 mmHg. Then, they investigated CPP when TCD indicated a lack of CBF and clinical BD was established. CPP was positive when clinical BD was present, and CBF was absent in 12 cases. Mean CPP at clinical manifestation of BD was 10.0 mmHg (range: 0-20 mmHg); mean CPP with cessation of CBF was 7.5 mmHg (0-20 mmHg). In 4 cases, clinical BD occurred before CBF decreased. The average CPP difference between clinical BD and CBF decrease was 8.8 mmHg (5-15 mmHg). It was possible to achieve CrCP even though CPP remained positive, which would have resulted in the loss of all CBF and BD. Neuromonitoring, which uses TCD at the bedside, could assist in the early identification of those at risk for CBF loss and, ultimately, BD.