OBJECTIVE: To investigate the specificity of in vivo amyloid imaging with [C]-Pittsburgh Compound B (PIB) in Parkinson disease dementia (PDD). METHODS: We performed detailed neuropathologic examination for 3 individuals with PDD who had PIB PET imaging within 15 months of death. RESULTS: We observed elevated cortical uptake of [C]-PIB on in vivo PET imaging in 2 of the 3 cases. At autopsy, all 3 individuals had abundant cortical Lewy bodies (Braak PD stage 6), and were classified as low-probability Alzheimer disease (AD) based on NIA-Reagan criteria. The 2 PIB-positive individuals had abundant diffuse Aβ plaques but only sparse neuritic plaques and intermediate neurofibrillary tangle pathology. The PIB-negative individual had rare diffuse plaques, no neuritic plaques, and low neurofibrillary tangle burden. CONCLUSIONS: [C]-Pittsburgh Compound B (PIB) PET is specific for fibrillar Aβ molecular pathology but not for pathologic diagnosis of comorbid Alzheimer disease in individuals with Parkinson disease dementia. The ability to specifically identify fibrillar Aβ amyloid in the setting of α-synucleinopathy makes [C]-PIB PET a valuable tool for prospectively evaluating how the presence of Aβ amyloid influences the clinical course of dementia in patients with Lewy body disorders.