Blake's pouch cyst is a rare posterior fossa cystic lesion characterized by posterior ballooning of the superior medullary velum into the cisterna magna. It must be differentiated from severe malformations like inferior vermian hypoplasia and Dandy Walker malformation. We describe a case in which a diagnosis of Blake's pouch cyst was made on prenatal ultrasound and later confirmed by MRI. The cyst showed complete regression on postnatal MRI.