Lewy bodies in Parkinson' s disease (PD) are strongly immunoreactive with antibodies against α-synuclein, which is mutated in some familial cases of the disease. We carried out immunohistochemical examinations of the brains of multiple system atrophy (MSA) patients using anti-α-synuclein antibodies. Strong α-synuclein immunoreactivity was found in glial cytoplasmic inclusions (GCIs), which are of oligodendroglial origin and occur exclusively in MSA. α-Synuclein-immunoreactive neuronal cytoplasmic inclusions (NCIs) were also found occasionally in the substantia nigra, pontine and inferior olivary nuclei, and dentate fascia. These findings indicate that α-synuclein is also a major component of GCIs and NCIs in MSA and strongly suggest that α-synuclein aggregation is a common process in certain neurodegenerative diseases, including PD and MSA.