In a 19-month-old patient with mucopolysaccharidosis I (Pfaundler-Hurler, MPS I/H) high-pressure hydrocephalus required the implantation of a ventriculo-peritoneal shunt. Despite a reduction in both ventricular volume and intracranial pressure, clinical symptoms suggesting compression of the brain stem persisted. Brain MRI revealed a tumor within the posterior cranial fossa. Cytologic examination of the cerebrospinal fluid was suggestive of a poorly differentiated ependymoma. High-pressure hydrocephalus is a common complication in MPS I/H. As changes in mucopolysaccharide metabolism may be associated with an increased risk of developing neoplasms, the possibility of an intracranial tumor should be considered in patients with MPS I/H and high-pressure hydrocephalus.