Filamentous inclusions made of α -synuclein constitute the defining neuropathological characteristic of Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy. Rare familial cases of Parkinson's disease are associated with mutations A53T and A30P in α -synuclein. We report here the assembly properties and secondary structure characteristics of recombinant α -synuclein. Carboxy-terminally truncated human α -synuclein (1-87) and (1-120) showed the fastest rates of assembly, followed by human A53T α -synuclein, and rat and zebra finch α -synuclein. Wild-type human α -synuclein and the A30P mutant showed slower rates of assembly. Upon shaking, filaments formed within 48 h at 37 degrees C. The related proteins β - and γ -synuclein only assembled after several weeks of incubation. Synthetic human α -synuclein filaments were decorated by an antibody directed against the carboxy-terminal 10 amino acids of α -synuclein, as were filaments extracted from dementia with Lewy bodies and multiple system atrophy brains. Circular dichroism spectroscopy indicated that α -synuclein undergoes a conformational change from random coil to β -sheet structure during assembly. X-ray diffraction and electron diffraction of the α -synuclein assemblies showed a cross-β conformation characteristic of amyloid.