Objectives To examine the association between marital status and dementia in a cohort of young-old (50–64) and middle-old (65–74) adults, and also whether this may differ by gender.

Design Prospective population-based study with follow-up time of up to 10 years.

Setting Swedish national register-based study.

Participants 2 288 489 individuals, aged 50–74 years, without prior dementia diagnosis at baseline. Dementia was identified using the Swedish National Patient Register and the Cause of Death Register.

Outcome measures The influence of marital status on dementia was analysed using Cox proportional hazards models, adjusted stepwise for multiple covariates (model 1: adjusted for age and gender; and model 2: additionally adjusted for having adult children, education, income and prior cardiovascular disease).

Results During follow-up, 31 572 individuals in the study were identified as demented. Cox regression showed each non-married subcategory to be associated with a significantly higher risk of dementia than the married group, with the highest risk observed among people in the young-old age group, especially among those who were divorced or single (HRs 1.79 vs 1.71, fully adjusted model). Analyses stratified by gender showed gender differences in the young-old group, with indications of divorced men having a higher relative risk compared with divorced women (HRs 2.1 vs 1.7, only-age adjusted model). However, in the fully adjusted model, these differences were attenuated and there was no longer any significant difference between male and female participants.

Conclusions Our results suggest that those living alone as non-marrieds may be at risk for early-onset and late-onset dementia. Although more research is needed to understand the underlying mechanism by which marital status is associated with dementia, this suggests that social relationships should be taken seriously as a risk factor for dementia and that social-based interventions may provide an opportunity to reduce the overall dementia risk.