Introduction: There is growing evidence of an aetiological relationship between vascular risk factors and the development of dementia in later life. Dementia in the under-65s has historically been considered to be more driven by genetic factors, but previous epidemiological studies in the young have been relatively small. This study aims to determine the prevalence of vascular comorbidity in people aged <65 with dementia in comparison to the general population. Methods: Analysis of routine clinical data from 314 (30%) general medical practices in Scotland. Results: From an overall population of 616 245 individuals, 1061 cases of 'all-cause' dementia were identified (prevalence 172/100 000 population, 95% CI 161 to 182). The prevalence of dementia was higher in people with vascular morbidities, and prevalence progressively increased from 129/100 000 in people with no vascular comorbidity to 999/100 000 in people with four or more (p=0.01). The strength of association was greatest with a previous transient ischaemic attack (TIA) or stroke and chronic kidney disease (adjusted OR=3.1 and 2.9, respectively). Statistically significant, but smaller associations were seen with the presence of hypertension, diabetes, ischaemic heart disease and peripheral vascular disease (adjusted OR=1.4, 2.0, 1.9 and 2.2, respectively). Discussions: Vascular comorbid diseases were more commonly recorded in people aged 40-64 with dementia than those without. This finding indicates that vascular disease may be more important in the aetiology of young-onset dementia than previously believed, and is of concern given the continuing rise in obesity and diabetes internationally.