A more sophisticated measure of abnormal cerebrovascular haemodynamics involves the transient hyperaemic response test (THRT) [Giller 1991]. This is used routinely in our department where we have demonstrated a relationship between an abnormal THRT and clinical outcome following SAH [Lam 2000]. The THRT assesses the FV response to a manual carotid compression (Figure 1) and can be repeated within a few minutes providing a reliable and readily accessible method for assessing autoregulation. When THRT is used for day-to-day assessments for the same patient, this test can provide a valuable monitor of sequential changes in autoregulation.