Passive immunization of murine models of Alzheimer disease amyloidosis reduces amyloid-β peptide (Aβ) levels and improves cognitive function. To specifically address the role of Aβ oligomers in learning and memory, they generated a novel monoclonal antibody, NAB61, that preferentially recognizes a conformational epitope present in dimeric, small oligomeric, and higher order Aβ structures but not full-length amyloid-β precursor protein or C-terminal amyloid-β precursor protein fragments. NAB61 also recognized a subset of brain Aβ deposits, preferentially mature senile plaques, and amyloid angiopathy. Using NAB61 as immunotherapy, this study showed that aged Tg2576 transgenic mice treated with NAB61 displayed significant improvements in spatial learning and memory relative to control mice. These data implicated Aβ oligomers as a pathologic substrate for cognitive decline in Alzheimer disease.