Background <p>Niemann–Pick disease type C (NP-C) is a rare genetic lysosomal lipid storage disorder characterized by progressive neurological impairment. Early diagnosis is critical for initiating treatment with miglustat, which can decelerate disease progression. In this study, we evaluated a cohort of 22 NP-C patients who underwent MRI, [¹⁸F]FDG PET, and clinical assessment at baseline. We performed a cross-sectional and longitudinal imaging study evaluating the role of [¹⁸F]FDG PET as an adjunct diagnostic tool for NP-C alongside MRI, the current neuroimaging standard.</p> Results <p>Group-level MRI analysis identified significant cerebellar and thalamic atrophy (d = 1.56, <i>p</i> &lt; 0.0001 and d = 1.09, <i>p</i> &lt; 0.001, respectively), with less pronounced involvement of the frontal lobe and hippocampus, which aligned with existing neuropathological understanding and guidelines. Conversely, [¹⁸F]FDG PET imaging revealed extensive hypometabolism in the cerebellum, thalamus, and cingulate cortex (d = 1.42, <i>p</i> &lt; 0.0001), and moderate hypometabolism in broad frontotemporal areas. [¹⁸F]FDG PET provided higher effect sizes across all brain regions, including regions without apparent atrophy, which suggests that it may be more sensitive than MRI for detecting NP-C neurodegenerative changes. Single-subject visual assessment of individual PET images further validated the clinical utility of [¹⁸F]FDG PET, with significant hypometabolism observed in the cerebellum, thalamus and anterior and posterior cingulate reported by physicians in 17/22 patients. Both hypometabolism and atrophy in the cerebellum were associated with ataxia, (more strongly indicated by [¹⁸F]FDG PET, <i>p</i> &lt; 0.0001 vs. MRI, <i>p</i> = 0.07). Medial temporal lobe atrophy was associated with cognitive impairment (<i>p</i> &lt; 0.05), and frontal hypometabolism was slightly related to behavioural impairment (<i>p</i> &lt; 0.07). Longitudinal [¹⁸F]FDG PET analysis revealed progressive subcortical, cortical and cerebellar hypometabolism, which was most pronounced in the cerebellum (-12% per year, <i>p</i> &lt; 0.001). Patients treated with miglustat showed a trend towards attenuated cerebellar hypometabolism progression compared to untreated patients (<i>p</i> = 0.10).</p> Conclusions <p>Our findings delineate a discernible hypometabolism pattern specific to NP-C that distinguishes it from other neurodegenerative conditions, thus suggesting that [¹⁸F]FDG PET might be a promising tool for NP-C diagnosis and to study disease progression.</p> Trial registration <p>XUNTA 2015/140. Registered 21 April 2015.</p>