<p>Chiari Malformation Type 1 (CM1) is canonically defined by ectopic position of the cerebellar tonsils with additional anatomic variations described inconsistently. Effect on fourth ventricle volume is controversial with prior studies reporting disparate results and methodologically limited to single institution series that hinder generalizability. This limitation was addressed utilizing multiple data sources, longitudinal replication, and reproducible methods using both FreeSurfer and the deep learning-based DL+DiReCT tools for automated volumetrics. First, we analyzed a local retrospective clinical cohort of individuals with CM1 and controls. Using data from the Adolescent Brain Cognitive Development (ABCD) Study, we analyzed volumes at baseline then replicated at two longitudinal timepoints. We then utilized an independent deep learning tool to demonstrate reproducibility with ABCD baseline data. Next, we again replicated with a prospective cohort from the Redefining Chiari (RC) study and compared to controls from the Human Connectome Project Young Adult (HCP-YA) study. Finally, we analyzed a heterogenous dataset from the Park-Reeves Syringomyelia Research Consortium (PRSRC) with comparison to ABCD baseline controls. Our aim was to test the hypothesis of a relationship between fourth ventricle size and CM1. Across all datasets, timepoints, and segmentation tools, we consistently found CM1 associated with smaller fourth ventricle volume. Our findings robustly demonstrate that a smaller fourth ventricle volume is an anatomical feature associated with CM1 at the group level. Fourth ventricle volume in CM1 may provide additional insights into pathophysiology but will require further study to fully elucidate its clinical importance.</p>