Background: Dementia is a major cause of disability, and risk-factor reduction may have the potential to delay or prevent the disease. Our aim was to determine the absolute 10-year risk of dementia, by age, sex and apolipoprotein E (APOE) genotype. Methods: We obtained data from the Copenhagen General Population Study (from 2003 to 2014) and the Copenhagen City Heart Study (from 1991 to 1994 and 2001 to 2003). Participants underwent a questionnaire, physical examination and blood sampling at baseline. Diagnoses of dementia and cerebrovascular disease were obtained from the Danish National Patient Registry up to Nov. 10, 2014. Results: Among 104 537 individuals, the absolute 10-year risk of Alzheimer disease in 3017 women and men who were carriers of the APOE ϵ44 genotype was, respectively, 7% and 6% at age 60- 69 years, 16% and 12% at age 70- 79 years, and 24% and 19% at age 80 years and older. Corresponding values for all dementia were 10% and 8%, 22% and 19%, and 38% and 33%, respectively. Adjusted hazard ratios (HRs) for all dementia increased by genotype, from genotype ϵ22 to ϵ32 to ϵ33 to ϵ42 to ϵ43 to ϵ44 (p for trend < 0.001). Compared with ϵ33 carriers, ϵ44 carriers were more likely to develop Alzheimer disease (adjusted HR 8.74, 95% confidence interval [CI] 7.08-10.79), vascular dementia (adjusted HR 2.87, 95% CI 1.54-5.33), unspecified dementia (adjusted HR 4.68, 95% CI 3.74-5.85) and all dementia (adjusted HR 5.77, 95% CI 4.89-6.81). Interpretation: Age, sex and APOE genotype robustly identify high-risk groups for Alzheimer disease and all dementia. These groups can potentially be targeted for preventive interventions.