BACKGROUND: The present study investigated the relationship between cardiorespiratory fitness and executive functioning in pediatric brain tumor survivors who received cranial radiation. This population is known to show executive dysfunction and lower rates of aerobic exercise compared to peers. PROCEDURE: Nine adolescent survivors of pediatric posterior fossa tumor completed an n-back working memory task during a functional MRI scan, as well as cardiorespiratory fitness testing on a cycle ergometer. RESULTS: Neuroimaging findings indicated typical activation patterns associated with working memory, mainly in the frontal-parietal network. Higher cardiorespiratory fitness was related to better performance on a behavioral measure of working memory and more efficient neural functioning. CONCLUSIONS: This study provides preliminary evidence that cardiorespiratory fitness may be related to executive functioning, particularly working memory, in pediatric brain tumor survivors. Descriptions of the brain regions recruited for working memory by pediatric brain tumor survivors may be used to inform future interventions or indicators of treatment efficacy.