Introduction. There is currently great interest in the relationship between physical activity (PA) and anterior hippocampal volume, with cross-sectional magnetic resonance imaging (MRI) studies indicating that higher PA levels are associated with greater volumes1,2, and interventional studies demonstrating that improving PA levels can lead to increases in volume3,4. However, replication of such positive findings in independent samples, the gold standard of the scientific process, is vital. Here, we use voxel-based morphometry (VBM) to examine the relationship between PA levels and anterior hippocampal volume in a cross-sectional study of members of the Whitehall II Imaging Sub-Study5 (Study 1) and, in a separate study, explore whether a twelve-week PA program can lead to increases in anterior hippocampal volume (Study 2).Methods. Study 1 included 359 participants (75 females, age: 68.7 ± 5.0 years). PA was assessed using the CHAMPS questionnaire, with met.min in moderate-to-vigorous PA subsequently categorized into tertiles representing low, medium and high PA levels. Mean anterior hippocampal volume was calculated from T1-weighted MRI scans using FSL-VBM. An ANCOVA, with age and gender as covariates, was performed to test if anterior hippocampal volume differed with PA level.Study 2 included 48 participants (30 females, age 66.7 ± 5.3) who were randomly assigned to supervised aerobic exercise sessions three times per week for 12-weeks or a waitlist. At baseline and after 12-weeks, PA was assessed using the CHAMPS questionnaire (met.min in moderate-to-vigorous PA) and mean anterior hippocampal volume calculated using FSL-VBM. ANCOVAs, with age and gender as covariates, were performed to examine if change in PA or change in anterior hippocampal volume differed between exercise and waitlist groups.Results. In Study 1, anterior hippocampal volume was not significantly different between groups (Figure 1A; F = 0.52, p = 0.597). In Study 2, the PA intervention was associated with a significant increase in both PA levels (Figure 1B, F = 5.4, p = 0.024) and anterior hippocampal volume (Figure 1C, F = 4.3, p = 0.045).Conclusions. Our results partly support the hypothesis that PA is associated with anterior hippocampal volume; however further work is required to explore the influence of age, gender and genotype on this relationship.