Fibroblast growth factor receptor mutations are associated with and, in fact, cause most syndromes presenting with craniosynostosis. This knowledge has resulted in a shift in the paradigm of suture fusion causation; it was thought previously that abnormal tensional forces arising in the cranial base caused fusion of the vault sutures, but it is now understood that aberrant intercellular signaling in the developing skull leads to abnormal suture morphogenesis. Although the mutations associated with these syndromes are known and the phenotypic consequences are well documented, the pathway from mutation to phenotype has yet to be elucidated. Surgical reconstruction is the primary treatment of craniofacial abnormalities associated with craniosynostotic syndromes such as Crouzon syndrome. In many cases, calvarial vault reshaping is dependent on the quality of the autologous bone available; however, the bone of patients with craniosynostosis syndrome is often more brittle, thinner, and less robust than cranial bone from nonaffected donors. The relation between syndromic craniosynostoses and this bone has not been previously described. In this study, the osteon and blood vessel diameters of calvarial bone from patients with Crouzon syndrome and age- and sex-matched normal calvarial bone are measured. Statistical analysis demonstrates a quantitative and significant difference in the blood vessel diameter but not in the osteon diameter. This finding could be a result of abnormal blood vessel development caused by the fibroblast growth factor receptor mutation occurring before and coincident with bone formation and leading to weakened and fragile bone tissue.