The Arctic (p. E693G) mutation in the amyloid-ß precursor protein (AßPP) facilitates amyloid-ß (Aß) protofibril formation and generates clinical symptoms of Alzheimer's disease (AD). Here, molecular details of Aß in post mortem brain were investigated with biochemical and morphological techniques. The basic structure of Arctic plaques resembled cotton wool plaques. However, they appeared ring-formed with Aß42-specific antibodies, but were actually targetoid, since the periphery and center of many parenchymal Aß deposits stained differently with mid-domain, N- and C-terminal Aß antibodies. Aß fibrils were similar in shape, albeit shorter than in sporadic AD brain, when examined by electron microscopy. Aßwild-type and Aßarctic codeposited and parenchymal deposits were highly enriched in both N- and C-terminally truncated Aß. In contrast, cerebral amyloid angiopathy (CAA) contained a substantial amount of Aß1-40. The absence of plaques with cores of fibrillary Aß might be due to the scarcity of full-length Aß, although other mechanisms could be involved. Our findings are discussed in relation to mechanisms and relevance of amyloid formation and to the clinical features of AD.