Abnormal accumulation and dysfunction of synaptic proteins play an important role in the pathogenesis of several neurodegenerative disorders. Lewy bodies, the fibrous portion of which contains the protein α-synuclein, can be neurotoxic, analogous to the proposed toxicity of amyloid plaques in Alzheimer disease. Lewy bodies are found in degeneration of the brain stem and cortical neurons in Parkinson's disease and in dementia with Lewy bodies, which is the second most common cause of cognitive impairment in patients with dementia. Recent studies have shown that Lewy bodies are also present in multiple system atrophy. Dementia with Lewy bodies is characterised by attentional deficits, frontal-subcortical dysfunction, visual hallucinations, Parkinsonism, and fluctuation of cognition. Dementia with Lewy bodies is amenable to treatment with cholinesterase inhibitors.