BACKGROUND/AIMS: Alleles of the FMR1 gene containing small expansions of the CGG-trinucleotide repeat comprise premutation and grey-zone alleles. Premutation alleles may cause late-onset Fragile X-associated tremor/ataxia syndrome attributed to the neurotoxic effect of elevated FMR1 transcripts. Our earlier data suggested that both grey-zone and low-end premutation alleles might also play a significant role in the acquisition of the parkinsonian phenotype due to mitochondrial dysfunction caused by elevated FMR1 mRNA toxicity. These data were obtained through clinical and molecular comparisons between carriers of grey-zone/low-end premutation alleles and group-matched non-carrier controls from patients with idiopathic Parkinson's disease (iPD). We aimed to explore the relationship between grey-zone alleles, parkinsonism and white matter changes. METHODS: This study compared the extent and severity of white matter hyperintensity (WMH) on magnetic resonance imaging, using a semi-quantitative method, between 11 grey-zone/low-end premutation carriers and 20 non-carrier controls with iPD from our earlier study. Relationships between WMH scores, and cognitive and motor test scores were assessed for carriers and non-carriers. RESULTS: Supratentorial WMH scores, and tremor and ataxia motor scores were significantly higher in carriers compared with disease controls. Moreover, some associations between cognitive decline and WMH scores were specific for each respective carrier status category. CONCLUSIONS: The results support our earlier claim that grey-zone alleles contribute to the severity of parkinsonism and white matter changes.