This amino acids 11 to 22 fragment of b-amyloid is capable of forming well-organized amyloid fibrils in vitro, similar to the pathogenic ones found in amyloidosis. Analysis of the structural properties of one monomer of b-amyloid (11-22) as well as of the aggregation mechanisms for four chains of b-amyloid (11-22) showed that the system assembles rapidly into a random globular state that evolves into three- and four-stranded antiparallel beta-sheets. The aggregation process is considerably accelerated by the presence of preformed dimers.