Malformations of cortical development (MCDs) comprise a variable spectrum of clinical, neuroradiologic, and histopathologic findings. MCDs are increasingly recognized as significant causes of epilepsy, developmental delays, and congenital neurologic deficits. The aim of this study was to determine the types, relative frequencies, and clinical and imaging features of MCDs. Data were collected in 2 hospitals and a medical imaging center during a 9-year period. Twenty-six patients (17 men and 9 women; age range, 5 mo to 29 y; mean age, 10.8 y) with an MCD were evaluated. The results of magnetic resonance imaging studies were retrospectively reviewed for type, extension, and classification of the malformations and for associated findings. Clinical findings were obtained by a review of the patients' medical records. Of the patients studied, epilepsy was present in 65%, mental and/or motor retardation was identified in 34.6%, and skin lesions were noted in 15%. The following types of MCD were identified: malformations of the heterotopic gray matter in 35% of the subjects, focal cortical dysplasia (23%), subependymal and cortical tubers (19%), pachygyria (15%), polymicrogyria (15%), schizencephaly (15%), and type 1 lissencephaly (8%). Approximately, 27% of the subjects had the following other types of cerebral malformation: callosal agenesis (8%), ventriculomegaly (8%), or agenesis of the septum pellucidum (4%). Our study indicated that the most common forms of MCD are heterotopia and focal cortical dysplasia. Patients with an MCD tended to have a higher prevalence of epilepsy, developmental delays, and neurologic deficits. Most patients with heterotopia had other malformations of cortical dysplasia. Some patients with an MCD also exhibited other malformations of the brain.