Background: The evaluation for epilepsy surgery of patients with malformations of cortical development (MCDs) in areas of clinically important cerebral function is a challenge because of the unpredictable localization of critical sensory, motor, and cognitive function. Magnetoencephalography (MEG) source localization of evoked fields can address whether functional reorganization of primary sensory modalities exists in MCDs. Methods: Consecutive patients with MRI- demonstrated rolandic and calcarine cortex MCDs were identified who had a 148- channel whole- head MEG study to identify the localization of primary somatosensory and visual cortices. Reorganization was considered when localization contrasted that expected upon general anatomic or homuncular rules and was defined against controls. Results: Twelve patients (n = 12) were studied. Six had focal cortical dysplasia, two had polymicrogyria and schizencephaly, and four had isolated polymicrogyria. In the patients with cortical dysplasias, the somatosensory cortices were identified outside the rolandic area. In the two patients with polymicrogyria and schizencephaly, the somatosensory cortices remained in the rolandic areas as long as the anatomy was not distorted by the presence of the schizencephalic cleft. In the patients with isolated polymicrogyria, the somatosensory cortex was mapped without evidence of reorganization. Conclusion: In patients with epileptic MCDs involving rolandic and calcarine regions, cortical function may be reorganized if the MCDs are due to an abnormal neuronal or glial proliferation (i.e., cortical dysplasia) but may not be in MCDs caused by abnormal cortical organization (i.e., polymicrogyria). Background: The evaluation for epilepsy surgery of patients with malformations of cortical development (MCDs) in areas of clinically important cerebral function is a challenge because of the unpredictable localization of critical sensory, motor, and cognitive function. Magnetoencephalography (MEG) source localization of evoked Methods: Consecutive patients with MRI-demonstrated rolandic and calcarine cortex MCDs were identified who had a 148- channel whole-head MEG study to identify the localization of primary somatosensory and visual cortices Reorganization was considered when localization contrasted that expected upon general anatomic or homuncular rules and was defined against controls. Results: Twelve patients (n = 12) were studied. Six had focal cortical dysplasia, two had polymicrogyria and schizencephaly, and four had isolated polymicrogyria In the patients with cortical dyspla sias, the somatosensory cortices were identified outside the rolandic area. In the two patients with polymicrogyria and schizencephaly, the somatosensory cortices remained in the rolandic areas as long as the anatomy was not distorted by the presence of the schizencephalic cleft. In the patients with isolated In patients with epileptic MCDs involving rolandic and calcarine regions, the cortical function may be reorganized if the MCDs are due to an abnormal neuronal or glial proliferation (ie, cortical dysplasia) but may not be in MCDs caused by abnormal cortical organization (ie, polymicrogyria).