A number of neurodegenerative diseases including Parkinson's disease, dementia with Lewy bodies (DLB) and multiple system atrophy are characterized by the formation and intraneuronal accumulation of fibrillar aggregates of alpha-synuclein (alpha-syn) protein in affected brain regions. These and other findings suggest that the accumulation of alpha-syn in the brain plays an important role in the pathogenesis of these diseases. However, more recently it has been reported that early amyloid aggregates or 'soluble oligomers' are the pathogenic species that lead to neurodegeneration and neuronal cell death rather than the later 'mature fibrils'. In this study, we investigated the presence of alpha-syn oligomers in brain lysates prepared from frozen post-mortem brains of normal, Alzheimer's disease and DLB patients. The brain extracts were subjected to high speed centrifugation, to remove insoluble alpha-syn aggregates, followed by specific detection of soluble oligomers in the supernatants by employing FILA-1, an antibody that specifically binds to alpha-syn aggregates, but not to alpha-syn monomers, or to tau or beta-amyloid aggregates. Using this novel enzyme-linked immunosorbent assay (ELISA) method to quantify the amounts of alpha-syn oligomers in the brain extracts, our data clearly show an increase in the levels of soluble oligomers of alpha-syn in the DLB brains compared to those with Alzheimer's disease and the controls (P < 0.0001). Our findings provide strong evidence to support the contention that elevated soluble oligomers of alpha-syn are involved in the pathogenesis of DLB. Furthermore, these findings establish FILA-1 as a very sensitive tool for the detection of oligomeric forms of alpha-syn in human brain lysates.