Objective: Patients with craniosynostosis syndromes are at risk of increased intracranial pressure (ICP) and Chiari I malformation (CMI), caused by a combination of restricted skull growth, venous hypertension, obstructive sleep apnea (OSA), and an overproduction or insufficient resorption of cerebrospinal fluid. This study evaluates whether craniosynostosis patients with CMI have an imbalance between cerebellar volume (CV) and posterior fossa volume (PFV), that is, an overcrowded posterior fossa. Methods: Volumes were measured in 3D-SPGR TI-weighted MR scans of 28 'not-operated' craniosynostosis patients (mean age: 4.0 years; range: 0-14), 85 'operated' craniosynostosis patients (mean age: 8.0 years; range: 1-18), and 34 control subjects (mean age: 5.4 years; range: 0-15). Volumes and CV/PFV ratios were compared between the operated and not-operated craniosynostosis patients, between the individual craniosynostosis syndromes and controls, and between craniosynostosis patients with and without CMI. Data were logarithmically transformed and studied with analysis of covariance (ANCOVA). Results: The CV, PFV, and CV/PFV ratios of not-operated craniosynostosis patients and operated craniosynostosis patients were similar to those of the control subjects. None of the individual syndromes was associated with a restricted PFV. However, craniosynostosis patients with CMI had a significantly higher CV/PFV ratio than the control group (0.77 vs. 0.75; p = 0.008). The range of CV/PFV ratios for craniosynostosis patients with CMI, however, did not exceed the normal range. Conclusion: Volumes and CV/PFV ratio cannot predict which craniosynostosis patients are more prone to developing CMI than others. Treatment should focus on the skull vault and other contributing factors to increased ICP, including OSA and venous hypertension. (C) 2015 European Association for Cranio-Maxillo-Facial Surgery. Published by Elsevier Ltd. All rights reserved.