Pediatric cancer patients and survivors frequently report posttraumatic stress symptoms (PTSS), which are associated with variation in stress-sensitive brain regions, including the hippocampus. We examined the impact of a novel, four-week martial-arts-based meditative intervention on cancer-related PTSS in pediatric patients and survivors, and whether hippocampal volumes at baseline correlate with PTSS severity and/or changes in PTSS over time. PTSS did not significantly change from baseline to post-intervention. However, smaller hippocampal volume was correlated with more severe re-experiencing PTSS at baseline, and greater reductions in PTSS post intervention. Together, hippocampal volume may be a biomarker of PTSS severity and intervention response.