Objective. in brain perfusion studies, perfusion defects have been reported mainly in the temporal and parietal regions in patients with Alzheimer disease (AD), but it is known that the occipital cortex is partially preserved from metabolic defects in patients with AD, at least in the early stage of the disease. We therefore evaluated the reactivity of the posterior cerebral arteries during visual stimulation with transcranial Doppler ultrasonography (TDU). Methods. Fifteen patients with AD (mean age, 70 years), 12 patients with vascular dementia (VD) (mean age, 61.5 years), and 9 healthy control subjects (mean age, 58 years) were enrolled in the study. The reactivity of both posterior cerebral arteries during visual stimulation was measured with TDU. Reactivity was calculated by the differences in mean relative blood flow velocity (rBFv) between stimulation (vs) and rest (vr) divided by the resting value [rBFv = 100 x (vs - vr)/vr]. Results. Significant increases of vascular reactivity were obtained during visual stimulation for each group (P <.001). There was no significant right or left side difference for vascular reactivity within the groups except in VD,- therefore, values of the right and left sides were pooled for patients with AD and control subjects. Reactivity to visual stimulation was similar for the patients with AD (mean +/- SD, 38% +/- 2.2%) and control subjects (35% +/- 1.8%), but reactivity was found significantly decreased in both sides of patients with VD (25.7% +/- 2.1% for the left side and 24% +/- 2.6% for the right side) compared with the healthy control subjects and the patients with AD (P <.001). Conclusions. These results indicate that the occipital cortex is partially preserved in patients with AD and that functional TDU may be a valuable tool for differentiating VD and AD.