Twenty necropsy cases of the association of fetal encephalitis with porencephaly, hydranencephaly or polymicrogyria were reviewed including 5 from the authors' material. The latter include a basket brain, a porencephalic necrosis of recent date and a polymicrogyria in the formative state. The supratentorial lesions are often associated with cerebellar cortical dysplasias. The pathogenetic interdependence of encephalitis and hemispheric defects or malformation is discussed. Examples are given in which the infection appears to be secondary to a pre-existent hemispheric lesion. For others the hemispheric lesions appear to be secondary to encephalitis. In these the encephalitis may produce secondary lesions by a variety of pathogenetic mechanisms.