Abnormal aggregation of human ␣-synuclein in Lewy bodies and Lewy neurites is a pathological hallmark of Parkinson disease and dementia with Lewy bodies. Studies have shown that oxidation and nitration of ␣-synuclein lead to the formation of stable dimers and oligomers through dityrosine cross-linking. Previously we have reported that tyrosine-to-cysteine mutations, particularly at the tyrosine 39 residue (Y39C), significantly enhanced ␣-synuclein fibril formation and neurotoxicity. In the current study, we have generated transgenic mice expressing the Y39C mutant human ␣-synuclein gene controlled by the mouse Thy1 promoter. Mutant human ␣-synuclein was widely expressed in transgenic mouse brain, resulting in 150% overexpression relative to endogenous mouse ␣-synuclein. At age 9 -12 months, transgenic mice began to display motor dysfunction in rotarod testing. Older animals aged 15-18 months showed progressive accumulation of human ␣-synuclein oligomers, associated with worse motor function and cognitive impairment in the Morris water maze. By age 21-24 months, ␣-synuclein aggregates were further increased, accompanied by severe behavioral deficits. At this age, transgenic mice developed neuropathology, such as Lewy body-like ␣-synuclein and ubiquitin-positive inclusions, phosphorylation at Ser 129 of human ␣-synuclein, and increased apoptotic cell death. In summary, Y39C human ␣-synuclein transgenic mice show age-dependent, progressive neuronal degeneration with motor and cognitive deficits similar to diffuse Lewy body disease. The time course of ␣-synuclein oligomer accumulation coincided with behavioral and pathological changes, indicating that these oligomers may initiate protein aggregation, disrupt cellular function, and eventually lead to neuronal death.