Utilizing the UIC Vascular Laboratory Registry, we retrospectively analysed the significance of side to side middle cerebral artery (MCA) flow velocity differences. Side to side differences > 15 percent measured by transcranial Doppler were considered asymmetric. Asymmetric subjects had 5 times greater chance of having significant cervical carotid narrowing on either side on Duplex Doppler and a 3.7 times greater chance of having a stroke on brain CT or MRI. MCA flow velocity asymmetry is a marker for underlying carotid disease and stroke.