Several small molecule ligands for amyloid-beta (Abeta) fibrils deposited in brain have been developed to facilitate radiological diagnosis of Alzheimer's disease (AD). Recently, the build-up of Abeta oligomers (AbetaO) in brain has been recognized as an additional hallmark of AD and may play a more significant role in early stages. Evidence suggests that quantitative assessment of AbetaO would provide a more accurate index of therapeutic effect of drug trials. Therefore, there is an urgent need to develop methods for efficient identification as well as structural analysis of AbetaO. We found that some well established amyloid ligands, analogs of Congo red and thioflavin-T (ThT), bind AbetaO with high affinity and detect AbetaO in vitro and in vivo. Binding studies revealed the presence of binding sites for Congo red- and thioflavin-T-analogs on AbetaO. Furthermore, these ligands can be used for imaging intracellular AbetaO in living cells and animals and as positive contrast agent for ultrastructural imaging of AbetaO, two applications useful for structural analysis of AbetaO in cells. We propose that by improving the binding affinity of current ligands, in vivo imaging of AbetaO is feasible by a 'signal subtraction' procedure. This approach may facilitate the identification of individuals with early AD.