Idiopathic parkinsonism (Parkinson’s disease) makes up the largest diagnostic subgroup of patients with parkinsonism. Various hypotheses exist regarding the pathogenesis of idiopathic parkinsonism: these include genetic predilection, aging, environmental factors, oxidative stress, excitotoxicity, autoimmunity, and trauma. We suggest that the pathogenesis of idiopathic parkinsonism is likely to be multifactorial, deriving from environmental factor(s) acting upon a genetically predisposed individual. Because of the compelling evidence indicating common clinical and pathological findings in idiopathic parkinsonism, Alzheimer’s disease, and amyotrophic lateral sclerosis, we believe that these conditions result from pathological processes with more similarity than diversity. A primary glutamatergic cell neocortical abnormality provides an attractive unifying explanation which may explain the overlapping abnormalities found in idiopathic parkinsonism, Alzheimer’s disease, and amyotrophic lateral scle