Lewy body disease (LBD) refers to a heterogeneous group of disorders presenting with parkinsonism and Lewy body (LB) formation. Although the relationship between dementing syndromes with LBs, Parkinson's disease, and Alzheimer's disease is unclear, the former constitute a common form of degenerative dementia and may account for up to 20% of cases in the elderly. We recently demonstrated triplication of the alpha-synuclein gene as the cause of disease in the Spellman-Muenter kindred. Neuropathological examination of affected members of the kindred demonstrated extensive LB pathology consistent with diffuse LBD. We examined a large collection of pathologically confirmed LBD cases and found no evidence for multiplication of the alpha-synuclein gene, suggesting that this mechanism is not a common cause of LBD.