Substantial evidence from epidemiological, pathological, and clinical reports suggests that vascular factors are critical in the pathogenesis of Alzheimer's disease (AD), and changes in blood flow are currently the most reliable indicators of the disease. We previously reported that older APP23 transgenic (tg) mice have significant blood flow alterations correlated with structural modifications of blood vessels. For the present study, our objective was to analyze the age-dependent morphological and architectural changes of the cerebral vasculature of APP23 tg mice. To visualize the 3D arrangement of the entire brain vasculature, we used vascular corrosion casts. Already at young ages, when typically parenchymal amyloid plaques are not yet present, APP23 tg mice had significant alterations, particularly of the microvasculature, often accompanied by small deposits attached to the vessels. In older animals, vasculature abruptly ended at amyloid plaques, resulting in holes. Often, small deposits were sitting near or at the end of truncated vessels. Between such holes, the surrounding vascular array appeared more dense and showed features typical for angiogenesis. We propose that small amyloid aggregates associated with the microvasculature lead to morphological and architectural alterations of the vasculature, resulting in altered local blood flow. The characteristic early onset of vascular alterations suggests that imaging blood flow and/or vasculature architecture could be used as a tool for early diagnosis of the disease and to monitor therapies. amyloid precursor protein ͉ cerebral blood flow ͉ scanning electron microscopy ͉ vascular corrosion casting T he typical clinical picture of Alzheimer's disease (AD) includes a progressive decline of memory function, often accompanied by other clinical signs such as agitation, aggression, sleep disturbances, and social withdrawal. Nonetheless, brain autopsy is needed to positively confirm the diagnosis (1). A high density of neuritic plaques, neurofibrillary tangles, and vascular amyloid (A) is a characteristic neuropathological marker of AD (2, 3). Plaques and tangles in the neuropil may affect neuronal function and also contribute to the neuronal damage; however, it is unclear whether their incidence correlates with the clinical signs and symptoms of cognitive impairment characteristic of the disease (4). Evidence suggests that cerebrovascular pathologies, such as structural alterations (5), atherosclerotic lesions (6), and impaired hemodynamic responses (7), are early features of AD (for review, see ref. 8). Reduced blood flow has been reported as one of the most consistent physiological deficits in AD (9, 10); however, it remains unclear whether the reduced cerebral blood flow is a response to neuronal damage or a factor initiating the characteristic neuropathology. In vivo studies showed the effect of A on cerebral blood flow and vessel architecture in a mouse model for AD (11,12). In other models, cerebrovascular regulatory mechanisms, such as endothelium-depende...