This experiment used functional transcranial Doppler ultrasonography to demonstrate that blood flow velocity in the anterior cerebral artery is hyporeactive in Huntington’s disease (HD) patients during maze testing (P<0.05). These vascular changes are due to tracing, as opposed to problem solving, components of the maze task. Using logistic regression, the reactivity of the ACA during activation is able to categorize patients as being either gene negative, or positive, for HD (P=0.0007). The possible role that nitric oxide/peroxynitrite may play in this phenomena is discussed.