Some clinical reports and epidemiological data suggest that a virus may play a role in the etiology of Parkinson's disease (PD). Following intracerebral injection of a neurovirululent strain of influenza A virus into mice, the virus was found to be particularly localized in neurons of the substantia nigra and hippocampus. Although efforts to detect virus particles in the brains, or antibodies in the serum or CSF of patients with PD have been generally unsuccessful, recent immunohistochemical work has revealed the presence of complement proteins and the interferon-induced MxA in association with Lewy bodies and swollen neuronal processes. Although a viral etiology for PD is not now widely accepted, we proposed such an hypothesis. Neurovirulent influenza A virus is a candidate, but some other viruses or complex infection of these viruses may be responsible for the formation of Lewy bodies and the later death of nigral neurons.