PORENCEPHALY in the hydrocephalic infant is an aggressive but preventable process. Pendergrass and Perryman1have emphasized that porencephaly is a manifestation of atrophy but note that the patients may have progression of their symptoms. Farmer2prefers the term encephalomalacic porencephaly and attributes the occurrence of these cysts to that particular property of the young brain which permits the rapid total dissolution of parenchyma after a necrotizing lesion. This would seem to ascribe a passive role to the cyst—a cyst ex-vacuo. Acquired porencephaly is due to the destruction of cerebral tissue from various causes such as trauma, circulatory disturbances, or inflammatory or degenerative processes according to Barrett and Mendelsohn.3Serial ventriculograms in the cases to be presented seem to demonstrate that the cyst formation is an active, erosive process with resulting loss of normal cerebral tissue and displacement of the ventricular system. Report of Cases Case1.—This patient