We examined brains from Parkinson's disease and from dementia with Lewy bodies (LBs) by using antibodies to NACP/α-synuclein. Immunohistochemically, all of the antibodies against the amino-terminal region, NAC domain, and carboxyl-terminal region of NACP labeled not only LBs, pale bodies (PBs), and dystrophic neurites, but also fine thread-like structures in the neuronal perikarya (perikaryal threads) in the hypothalamus and brainstem nuclei. On electron microscopy, immunoreactive products were found to label the 9 to 12 nm-thick filamentous component (LB-filaments) of LBs, PBs, and perikaryal threads. The NACP-immunoreactive perikaryal threads, consisting of small bundles of LB-filaments and randomly oriented LB-filaments, presumably represent an initial stage of LB- or PB-formation. The present study indicates that the entire molecule of NACP is involved in the neuronal filament-aggregating processes of LB disorders.