A 1-month-old infant with a Dandy-Walker malformation underwent the placement of a ventriculoperitoneal shunt to treat progressive hydrocephalus. On the initial computed tomographic scan, a subependymal cystic lesion larger than 1 cm in diameter was noted in the head of the right caudate nucleus, which enlarged to 3 cm in diameter on follow-up imaging studies over the course of a year. Biopsies taken at the time of fenestration failed to show the presence of neoplasia. This appears to be the first reported case of a cyst of this nature to show progressive enlargement.