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Background: Huntington disease prevalence was first estimated in Grampian, northern Scotland in 1984. Molecular testing has since increased ascertainment., Objective: To estimate the prevalence of manifest Huntington disease and identified pre-symptomatic gene expansion carriers (IPGEC) in northern Scotland, and estimate the magnitude of biases in prevalence studies that rely upon routine coding in primary care records., Methods: Cases were ascertained using North of Scotland genetic laboratory, clinic, and hospital records. Prevalence was calculated for manifest and IPGEC on 01/07/2016 and 01/01/2020 and compared with local published data., Results: The prevalence of manifest Huntington disease in northern Scotland in 2020 was 14.6 (95% CI 14.3-15.3) per 100,000, and of IPGEC was 8.3 (95% CI 7.8-9.2) per 100,000. Whilst the population of northern Scotland decreased by 0.05% between 2016 and 2020, the number of manifest and identified pre-symptomatic gene expansion carriers increased by 7.4% and 23.3%, respectively. Manifest disease in Grampian increased by 45.9% between 1984 and 2020. More women than men had a diagnosis. General Practice coding underestimated symptomatic molecularly confirmed prevalence by 2.2 per 100,000 people., Conclusion: Even in an area with previously high ascertainment, there has been a 45.9% increase in manifest Huntington disease over the last 30 years. Within our catchment area, prevalence varies between health board regions with similar community-based services. Such variation in prevalence could have major drug cost and service delivery implications, especially if expensive, complexly administered therapies prove successful. Health services should gather accurate population-based data on a regional basis to inform service planning., (© 2021. The Author(s).)