The beta amyloid (A beta) peptide aggregates to form beta-rich structures that are known to trigger Alzheimer's disease. Experiments suggest that an alpha-helical intermediate precedes the formation of these aggregates. However, a description at the molecular level of the alpha-to-beta transition has not been obtained. Because it has been proposed that the transition might be initiated in the amino-terminal region of A beta, we studied the aggregation of the 28-residue amino-terminal fragment of A beta (A beta(1-28)) using molecular dynamics and a coarse-grained force field. Simulations starting from extended and helical conformations showed that oligomerization is initiated by the formation of intermolecular beta-sheets between the residues in the N-terminal regions. In simulations starting from the alpha-helical conformation, forcing residues 17-21 to remain in the initial (helical) conformation prevents aggregation but allows for the formation of dimers, indicating that oligomerization, initiated along the nonhelical N-terminal regions, cannot progress without the alpha-to-beta transition propagating along the chains.