A peptide derived from A beta(17-36) crystallizes to form trimers that further associate to form higher-order oligomers. The trimers consist of three highly twisted beta-hairpins in a triangular arrangement. Two trimers associate face-to-face in the crystal lattice to form a hexamer; four trimers in a tetrahedral arrangement about a central cavity form a dodecamer. These structures provide a working model for the structures of oligomers associated with neurodegeneration in Alzheimer's disease.