BACKGROUND: Cognitive deficits contribute to disability in Parkinson's disease (PD). Cognitive intra-individual variability (IIV) is associated with cognitive decline in age-related disorders, but IIV has not been related to functional ability in PD. We examined IIV in predicting functional ability in participants with PD. METHODS: De-identified National Alzheimer's Coordinating Center data ( RESULTS: PD participants showed lower OTBM and greater IIV, GDS, and motor dysfunction than controls ( CONCLUSION: IIV independently predicts functional status in participants with PD while controlling for other variables. PD participants with functional impairment at follow up showed greater IIV than those without functional impairment at follow up.