Objective: Congenital bilateral perisylvian syndrome(CBPS) has been defined as a characteristic malformative perisylvian polymicrogyria(PMG) in patients with clinical symptoms of pseudobulbar palsy and epileptic seizures. For the present study, we investigate clinicopathologic features of CBPS associated with timing of lesion formation. Methods: Clinicopathologic features of CBPS from 6 patients with surgical resection of the cerebral lesions due to medically intractable seizures were studied. Results: Seizure onset ranged from 1 to 10years (average 6.7years) of age, and average duration of seizure was 23years. All had complex partial seizures, and two patients had additional tonic clonic seizures. Magnetic resonance (MR) images showed polymicrogyria, atropic gyri with gliosis. In the histopathologic examination, the cortical lesions revealed features of ulegyria; atrophic and sclerotic gyri, laminar loss of neurons, extensive lobular gliosis throughout the gray and white matter, neuronoglial nodule formation, and many amyloid bodies. Unlayered or four-layered PMG was not identified. Conclusion: Above data suggest that CBPS might be caused by ulegyria resulting from developmental cortical defect during early fetal stage or acquired hypoxic/ischemic injury in prenatal or postnatal life.