Background and Purpose. Previous studies of transcranial Doppler (TCD) sonography in acute stroke have used the relative difference between the symptomatic and asymptomatic arteries to assess arterial occlusion. However, a simple measure of absolute mean flow velocity might provide a direct assessment of “perfusion reserve” in acute ischemic stroke. Methods. In a prospective study, 62 patients with ischemic stroke had TCD and a mean cerebral transit time examination within 48 hours of stroke. Absolute intracranial arterial mean flow velocities were correlated with the corresponding absolute mean transit times. Results. The authors found a significant correlation between middle cerebral artery (MCA) mean flow velocity and transit time in the symptomatic (Spearman rank correlation coefficient [rho]=-0.65, P<.01) but not in the asymptomatic (ρ=-0.04, P=ns) MCA territory, Equations relating absolute mean flow velocity to absolute transit time were derived. Conclusion, The findings suggest that in the normal hemisphere (with intact autoregulation on the horizontal portion of the autoregulation curve), flow velocity and transit time are not closely related to each other, but in the symptomatic hemisphere (on the downward slope of the autoregulation curve), flow velocity is directly proportional to the transit time and, therefore, to its inverse, perfusion reserve, The use of absolute mean flow velocity values on TCD should be further explored as a simple way of assessing "perfusion" in acute ischemic stroke.