Alzheimer's disease and Parkinson's disease are associated with the cerebral accumulation of β-amyloid and α-synuclein, respectively. Some patients have clinical and pathological features of both diseases, raising the possibility of overlapping pathogenetic pathways. We generated transgenic (tg) mice with neuronal expression of human β-amyloid peptides, α-synuclein, or both. The functional and morphological alterations in doubly tg mice resembled the Lewy-body variant of Alzheimer's disease. These mice had severe deficits in learning and memory, developed motor deficits before α-synuclein singly tg mice, and showed prominent age-dependent degeneration of cholinergic neurons and presynaptic terminals. They also had more α-synuclein-immunoreactive neuronal inclusions than α-synuclein singly tg mice. Ultrastructurally, some of these inclusions were fibrillar in doubly tg mice, whereas all inclusions were amorphous in α-synuclein singly tg mice. β-Amyloid peptides promoted aggregation of α-synuclein in a cell-free system and intraneuronal accumulation of α-synuclein in cell culture. β-Amyloid peptides may contribute to the development of Lewy-body diseases by promoting the aggregation of α-synuclein and exacerbating α-synuclein-dependent neuronal pathologies. Therefore, treatments that block the production or accumulation of β-amyloid peptides could benefit a broader spectrum of disorders than previously anticipated.