The authors report on two children in whom an anomalous posterior fossa venous drainage pattern prevented safe posterior fossa decompression. Both patients had Chiari malformation Type I, multisutural craniosynostosis, and crowded posterior fossa structures. Both patients had been treated with ventriculoperitoneal shunts for hydrocephalus. Pfeiffer syndrome had been diagnosed in one of the patients, and the other was suspected to have osteogenesis imperfecta. Although both patients were believed to have symptoms resulting from brainstem compression, posterior fossa decompression was not offered due to profound venous anomalies noted on imaging studies that greatly increased the expected risks associated with surgery. These cases are presented to alert neurosurgeons to carefully evaluate the posterior fossa venous anatomy prior to considering posterior fossa decompression with or without occipitocervical fusion or calvarial vault remodeling procedures in patients with multisutural craniosynostosis.