Background Huntington's disease (HD) is an inherited, progressive neurodegenerative disease that has no cure. Striatal atrophy and hypometabolism has been described in HD as far as 15 years before clinical onset and therefore structural and functional imaging biomarkers are the most applied biomarker modalities which call for these to be exact; however, most studies are not considering the partial volume effect and thereby tend to overestimate metabolic reductions, which may bias imaging outcome measures of interventions. Objective Evaluation of partial volume effects in a cohort of premanifest HD gene-expansion carriers (HDGECs). Methods 21 HDGECs and 17 controls had a hybrid 2-[.sup.18 F]FDG PET/MRI scan performed. Volume measurements and striatal metabolism, both corrected and uncorrected for partial volume effect were correlated to an estimate of disease burden, the CAG age product scaled (CAP.sub.S). Results We found significantly reduced striatal metabolism in HDGECs, but not in striatal volume. There was a negative correlation between the CAP.sub.S and striatal metabolism, both corrected and uncorrected for the partial volume effect. The partial volume effect was largest in the smallest structures and increased the difference in metabolism between the HDGEC with high and low CAP.sub.S scores. Statistical parametric mapping confirmed the results. Conclusions A hybrid 2-[.sup.18 F]FDG PET/MRI scan provides simultaneous information on structure and metabolism. Using this approach for the first time on HDGECs, we highlight the importance of partial volume effect correction in order not to underestimate the standardized uptake value and thereby the risk of overestimating the metabolic effect on the striatal structures, which potentially could bias studies determining imaging outcome measures of interventions in HDGECs and probably also symptomatic HD.