Background and objective <p>Numerous studies suggest that the development of Alzheimer’s Disease (AD) leads to a reduction in overall hippocampal volume. However, there is limited research exploring whether pre-morbid differences in hippocampal volume impact the risk of AD. This study aims to delve into the causal relationship between hippocampal subregional volume and AD using bidirectional Mendelian Randomization (MR) methods.</p> Methods <p>We extracted 44 instrumental variables for hippocampal subregional volume from the GWAS Catalog, involving 21,282 European individuals. Data on Alzheimer’s Disease were sourced from the Psychiatric Genomics Consortium, comprising 1,126,563 European individuals. Rigorous methods were employed to select instrumental variables, with the primary analysis conducted using the Inverse Variance Weighted method. Several sensitivity analyses included tests for heterogeneity, pleiotropy, and outliers. The obtained SNPs were mapped to genes for pathway enrichment analysis to explore the potential mechanisms underlying the regulation of hippocampal volume in Alzheimer’s disease.</p> Results <p>The study found significant causal associations between increased volume of the 5 hippocampal subfields with increased risk of AD. Conversely, increased Left hippocampus amygdala-transition-area volume was associated with reduced risk of AD. In reverse MR, AD was found to decrease the volume of 8 hippocampal subfields, while increasing the volume of the left hippocampal-fissure region. Amyloid-beta formation, leukocyte activation, and positive regulation of immune response mediated the changes in hippocampal subregional volume due to AD.</p> Conclusion <p>This MR study provides evidence that AD is causally related to hippocampal subfield volume, highlighting the roles of amyloid-beta formation and immune alterations in this context.</p>