Two cases are presented of absence of the septum pellucidum associated with bilateral polymicrogyria. In one case a circumscribed, completely enclosed cavity was present in the white matter of one cerebral hemisphere, different in structure from typical prenatal porencephaly. It is suggested that these cases represent a "forme fruste" of the syndrome of absent septum, bilateral porencephaly, polymicrogyria and heterotopia and may be ascribed to a similar, if less severe, encephaloclastic process of debatable etiology, operating around the midterm of pregnancy.