We investigated the clinical presentations and neuroimaging characteristics of five patients with sublobar dysplasia, an unusual malformation of cortical development. Records and teaching files of five unrelated patients with a localized dysplasia of the cerebral hemisphere separated from the remainder of the affected lobe or hemisphere by a deep infolding of cortex (sublobar dysplasia) were retrospectively reviewed with regard to age at clinical presentation, manner of clinical presentation, neurologic examination, location of dysplasia, imaging characteristics, and the presence and type of associated malformations. Four of five patients presented with seizures; the fifth presented with a calvarial anomaly. All were neurologically and developmentally normal. MRI showed that the areas of sublobar dysplasia were frontal in two patients and temporal, parietal, and occipital in one patient each. The cortex in the affected region of brain was thickened with shallow sulci and an abnormal sulcal pattern in all affected patients. In three patients, the cortical-white matter junction was irregular. The ipsilateral lateral ventricle was dysplastic in all patients. Associated anomalies included callosal anomalies (five patients), cerebellar vermian hypoplasia (three patients), and venous malformation (one patient). Sublobar dysplasia appears to be a distinct cortical malformation of unknown etiology that causes no neurologic deficits but ultimately results in epilepsy. Possible causes include abnormal stem cell proliferation and in utero injury.