Early interventions to improve resilience require the identification of objective risk biomarkers for PTSD symptom development. Although altered hippocampal and amygdala volumes are consistently observed in PTSD, it remains currently unknown whether they represent a predisposing vulnerability factor for PTSD symptom development or an acquired consequence of trauma exposure and/or the disorder. We conducted a longitudinal, prospective study in 210 police recruits at high risk for trauma exposure (56 females(26.7%); mean[SD] age = 24.02[5.19]). Structural MRI scans and trauma-related symptom severity were assessed at pre-trauma baseline and at 16-month follow-up. Between assessments, police recruits were exposed to various potentially traumatic events during their police training. Police recruits reported a significant increase in police-related trauma exposure and stress-related symptoms between assessments. Smaller hippocampal left dentate gyrus (DG) volumes at baseline predicted increase in self-reported PTSD symptoms (B[SE] = -0.21[0.08], p = 0.011), stress symptoms (B[SE] = -0.16[0.07], p = 0.024) and negative affect (B[SE] = -0.21[0.07], p = 0.005) upon trauma exposure. Amount of police-related trauma exposure between assessments was positively associated with an increase in left basal amygdala nucleus volume (B[SE] = 0.11[0.05], p = 0.026). Taken together, smaller DG-volumes pre-trauma may represent a predisposing neurobiological vulnerability factor for development of trauma-related symptoms. On the other hand, amount of trauma exposure between assessments was positively associated with increased amygdala basal nucleus volume, suggesting acquired neural effects. These findings suggest that preventive interventions for PTSD aimed at improving resilience could be targeted at increasing DG-volume and potentially its functioning.