Abnormal cerebrospinal fluid (CSF) collections within the posterior fossa are defined by the Dandy-Walker complex (DWC) and by arachnoid cysts (AC). The DWC includes the Dandy-Walker malformation (DWM), the Dandy-Walker variant (DWV) and the mega-cisterna magna (MCM). In addition, Tortori-Donati et al. added persistent Blake's pouch cyst (BPC) as an independent entity within the DWC. BPC represents a posterior ballooning of the superior medullary velum into the cisterna magna. All of these malformations are overlapping developmental anomalies characterized by varying degrees of malformation of the medullary vela, the cerebellar vermis and hemispheres, the fourth ventricle choroid plexus, the posterior fossa subarachnoid cisterns and the enveloping meningeal structures. We present two cases of persistent BPC detected in two adult women without history of gestational or subsequent growth problems. They underwent neuroradiological investigation because of headache and because of recurrent episodes of loss of consciousness, respectively. The MRI findings included tetraventricular hydrocephalus, wide communication of the fourth ventricle and the cystic posterior fossa (i.e. BPC), inferior posterior fossa mass effect with or without hypoplasia of both the cerebellar vermis and the medial aspects of the cerebellar hemispheres, and absence of communication between fourth ventricle and the basal subarachnoid space in the midline posteriorly. Persistent BPC is defined by a failure of embryonic assimilation of the area membranacea anterior within the tela choroidea associated with imperforation of the foramen of Magendie. Typically this condition becomes symptomatic early in life. In the current cases the normal function of the laterally positioned foramina of Luschka probably helped to maintain some CSF flow between intraventricular and subarachnoid spaces, with the establishment of a precarious equilibrium characterized by a compensatory enlargement of the cerebral ventricular system (i.e. hydrocephalus).