Two brothers and an unrelated male with neurofibromatosis (NF) developed hydrocephalus in childhood. Aqueductal stenosis (AS) was demonstrated by ventriculography and required shunt operation. Genetic counselors had to decide whether to invoke an X-linked gene for AS or whether AS could be caused by the NF mutation. Reports of 13 patients of both sexes with AS and NF suggest that AS with resulting hydrocephalus may be a rare manifestation of the NF gene. AS should be looked for in young NF patients with signs of hydrocephalus or with neurologic abnormalities.