Deposition of amyloid β-peptide (Aβ) in cerebral vessel walls (cerebral amyloid angiopathy, CAA) is very frequent in Alzheimer’s disease and occurs also as a sporadic disorder. Here, we describe significant CAA in addition to amyloid plaques, in aging APP/Ld transgenic mice overexpressing the London mutant of human amyloid precursor protein (APP) exclusively in neurons. The number of amyloid-bearing vessels increased with age, from ∼10 to >50 per coronal brain section in APP/Ld transgenic mice, aged 13 to 24 months. Vascular amyloid was preferentially deposited in arterioles and ranged from small focal to large circumferential depositions. Ultrastructural analysis allowed us to identify specific features contributing to weakening of the vessel wall and aneurysm formation, ie, disruption of the external elastic lamina, thinning of the internal elastic lamina, interruption of the smooth muscle layer, and loss of smooth muscle cells. Biochemically, the much lower Aβ42:Aβ40 ratio evident in vascular relative to plaque amyloid, demonstrated that in blood vessel walls Aβ40 was the more abundant amyloid peptide. The exclusive neuronal origin of transgenic APP, the high levels of Aβ in cerebrospinal fluid compared to plasma, and the specific neuroanatomical localization of vascular amyloid strongly suggest specific drainage pathways, rather than local production or blood uptake of Aβ as the primary mechanism underlying CAA. The demonstration in APP/Ld mice of rare vascular amyloid deposits that immunostained only for Aβ42, suggests that, similar to senile plaque formation, Aβ42 may be the first amyloid to be deposited in the vessel walls and that it entraps the more soluble Aβ40. Its ability to diffuse for larger distances along perivascular drainage pathways would also explain the abundance of Aβ40 in vascular amyloid. Consistent with this hypothesis, incorporation of mutant presenilin-1 in APP/Ld mice, which resulted in selectively higher levels of Aβ42, caused an increase in CAA and senile plaques. This mouse model will be useful in further elucidating the pathogenesis of CAA and Alzheimer’s disease, and will allow testing of diagnostic and therapeutic strategies.