We studied 12 patients with absence of the septum pellucidum associated with porencephalies. Seven patients had bilateral and five apparently unilateral defects of the cerebral mantle. The clinical manifestations were relatively mild in half of the patients and limited to isolated partial seizures in one. In one case, extensive calcification was present adjacent to the defects. In two cases, the presence of a cleft was demonstrated only by magnetic resonance imaging, although it had been suspected by computed tomography in one.