Vascular dementia (VD) and Alzheimer's dementia (AD) are the most common differential diagnoses in patients with cognitive impairment. Although of different etiology, small vessel disease is postulated to be present in both conditions. We investigated global cerebral blood flow (CBF), global cerebral circulation time (CCT) and global cerebral blood volume (CBV) in VD and AD patients using a multimodal ultrasound (US) approach. 20 VD and 20 AD patients were included and compared with 12 age-matched controls. Duplex US of both internal carotid and vertebral arteries was performed to measure CBF. CCT was defined as the time delay of an echo-contrast bolus arrival between the internal carotid artery and internal jugular vein using extracranial Doppler. CBV was calculated as the product of CBF and CCT. CBF was significantly lower (VD: 570 +/- 61; AD: 578 +/- 77; controls: 733 +/- 54 ml/min) and CCT significantly longer (8.8 +/- 2.6; 8.2 +/- 1.4; 6.4 +/- 0.8 s) in both patient groups compared with controls (p < 0.003). No difference in CBF and CCT was found between the two patient groups. CBV was similar in all three groups (82 +/- 20; 79 +/- 19; 78 +/- 9 ml). The equally reduced CBF and prolonged CCT in VD and AD support the hypothesis, that small vessel disease is a relevant factor in both types of dementia. The presented multimodal US approach helps to assess the extent of changes in the global cerebral hemodynamics in patients with dementia but does not allow a differentiation between VD and AD.