Epidemiologic evidence implicates cyclooxygenase activity in the pathogenesis of Alzheimer's disease, in which amyloid plaques have been found to contain increased levels of dimers and higher multimers of the amyloid β peptide. The product of the oxygenation of arachidonic acid by the cyclooxygenases, prostaglandin H₂ (PGH₂), rearranges non‐enzymatically to several prostaglandins, including the highly reactive γ‐keto aldehydes, levuglandins E₂ and D₂. We demonstrate that PGH₂ markedly accelerates the formation of dimers and higher oligomers of amyloid β₁₋₄₂. This is associated with the formation of levuglandin adducts of the peptide. These findings provide the molecular basis for a hypothesis linking cyclooxygenase activity to the formation of oligomers of amyloid β.