Dementia with Lewy bodies and Parkinson's disease dementia are different clinical phenotypes of Lewy body dementias differentiated by the temporal relationship between parkinsonism and dementia onset. At present, it is unclear whether the glutamatergic system is affected in these disorders. In this study, we measured α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor GluA subunits in the postmortem neocortex of a cohort of prospectively studied Lewy body dementia cases, as well as age-matched controls by immunoblotting. We found losses of GluA2/3/4 immunoreactivities in Lewy body dementias which correlated with higher pre-death Hoehn and Yahr scores and with longer Parkinson's disease duration before dementia onset, but not with dementia severity, cortical Lewy body burden, or amyloid plaque and neurofibrillary tangle burden. Our study suggests that GluA2/3/4 losses may be a neurochemical marker of motor disability in Lewy body dementias.