Porencephaly is defined as an area of focal encephalomalacia that communicates with the ventricular system, giving the appearance of a dilated ventricle. It is assumed that vascular occlusion or an infectious process cause gray and white matter necrosis leading to cyst formation in the cerebral parenchyma. Possible etiology of porencephaly includes intracerebral thrombosis, intracranial hemorrhage and fetal infection. We report the first antenatal diagnosis of porencephaly, which apparently resulted from maternal carbon monoxide poisoning.