We describe 9 patients with a bilateral malformation of cortical development, centered around the parasagittal and mesial aspects of the parietooccipital cortex, with magnetic resonance imaging findings suggestive of polymicrogyria. No familial distribution or etiologic factors were identified. Location in a watershed area between anterior and posterior cerebral arteries suggests postmigratory perfusion failure as the underlying cause. In most patients the malformation was detected by magnetic resonance imaging after computed tomography scans with 10-mm-thick sections were considered normal. Seizures, present in all, had started between the ages of 20 months and 15 years (mean, 9 years) and were intractable in 7. Complex partial seizures with or without minor automatisms were the most frequent ictal pattern. In only 4 patients these were preceded by symptoms indicating posterior onset. Interictal electroencephalograms showed both diffuse and bilateral parietooccipital or temporal abnormalities. The range of IQ scores indicated average intelligence to mild retardation. Several patients presented deficits on neuropsychological tasks requiring performance under time constraints, suggesting that the malformation may result in cognitive slowing. Early diagnosis of this malformation may be difficult because of the lack of neurological signs, relatively late seizure onset, difficulty in localizing seizure onset, and inability to recognize the cortical abnormality on computed tomography scans.