The amyloid peptides Abeta(40) and Abeta(42) of Alzheimer's disease are thought to contribute differentially to the disease process. Although Abeta(42) seems more pathogenic than Abeta(40), the reason for this is not well understood. We show here that small alterations in the Abeta(42):Abeta(40) ratio dramatically affect the biophysical and biological properties of the Abeta mixtures reflected in their aggregation kinetics, the morphology of the resulting amyloid fibrils and synaptic function tested in vitro and in vivo. A minor increase in the Abeta(42):Abeta(40) ratio stabilizes toxic oligomeric species with intermediate conformations. The initial toxic impact of these Abeta species is synaptic in nature, but this can spread into the cells leading to neuronal cell death. The fact that the relative ratio of Abeta peptides is more crucial than the absolute amounts of peptides for the induction of neurotoxic conformations has important implications for anti-amyloid therapy. Our work also suggests the dynamic nature of the equilibrium between toxic and non-toxic intermediates.