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Objective To test the hypothesis that low density lipoprotein (LDL) cholesterol due to genetic variation in the genes responsible for LDL cholesterol metabolism and biosynthesis (PCSK9 and 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR), respectively) is associated with a high risk of Alzheimer's disease, vascular dementia, any dementia, and Parkinson's disease in the general population.Design Mendelian randomisation study.Setting Copenhagen General Population Study and Copenhagen City Heart Study.Participants 111 194 individuals from the Danish general population.Main outcome measures Risk of Alzheimer's disease, vascular dementia, all dementia, and Parkinson's disease.Results In observational analyses, the multifactorially adjusted hazard ratio for Parkinson's disease in participants with an LDL cholesterol level <1.8 mmol/L versus ≥4.0 mmol/L was 1.70 (95% confidence interval 1.03 to 2.79), whereas the corresponding hazard ratios for Alzheimer's disease, vascular dementia, or any dementia did not differ from 1.0. PCSK9 and HMGCR variants combined were associated with a 9.3% lower LDL cholesterol level. In genetic, causal analyses adjusted for age, sex, and year of birth, the risk ratios for a lifelong 1 mmol/L lower LDL cholesterol level were 0.57 (0.27 to 1.17) for Alzheimer's disease, 0.81 (0.34 to 1.89) for vascular dementia, 0.66 (0.34 to 1.26) for any dementia, and 1.02 (0.26 to 4.00) for Parkinson's disease. Summary level data from the International Genomics of Alzheimer's Project using Egger Mendelian randomisation analysis gave a risk ratio for Alzheimer's disease of 0.24 (0.02 to 2.79) for 26 PCSK9 and HMGCR variants, and of 0.64 (0.52 to 0.79) for 380 variants of LDL cholesterol level lowering.Conclusion Low LDL cholesterol levels due to PCSK9 and HMGCR variants had no causal effect on high risk of Alzheimer's disease, vascular dementia, any dementia, or Parkinson's disease; however, low LDL cholesterol levels may have a causal effect in reducing the risk of Alzheimer's disease.