Clinical, radiological and pathomorphological investigations on five cases offering the patterns of the so-called temporal lobe agenesis syndrome (Robinson) have shown that this is not always originally malformative, but may be due to vascular processes in the sense of porencephalic defects, with all their pathomorphological features. We think that compression of the vein of Rosenthal by inscisural hippocampal herniation is the most probable aetiological mechanism. Earle, Baldwin and Penfield also described large temporal defects in connexion with their impressive demonstration of the inscisural herniation of the hippocampal gyrus at birth. We propose to use the name of--inscisural porencephaly--for this state.