Objectives: To determine the scope of histopathological variation in lesions responsible for epileptogenesis, I retrospectively reviewed the features of surgical specimens taken consecutively from 600 patients with intractable epilepsy. Methods: The patients were divided into three groups on the basis of age at seizure onset: 94 patients with infantile onset (before 1 year of age), 307 patients with juvenile onset (between 1 and 12 years of age), and 199 patients with adolescent/adult onset (at 13 years of age or beyond). Histological and immunohistochemical evaluations of the surgical specimens were performed. Results: In the infant group, seizure duration was significantly shorter than in the other groups, and malformations caused by abnormalities of cortical development, including focal cortical dysplasia (FCD) type IIa/b, tuberous sclerosis, hemimegalencephaly, and polymicrogyria were predominant, whereas in the juvenile and adolescent/adult groups, other lesions such as hippocampal sclerosis (HS), tumors, FCD type I, and vascular lesions were frequently observed. For patients with HS, seizure duration in the juvenile group was significantly longer than in the adolescent/adult group. FCD type IIIa was noted in nearly half of patient with HS in both juvenile and adolescent/adult groups. The causative tumors included dysembryoplastic neuroepithelial tumors, gangliogliomas, astrocytomas, and other glioneuronal and glial tumors. Conclusion: Various histopathological entities and types, showing clear predominance depending on the age at seizure onset, were observed in patients with epilepsy. These features appear to provide information on the pathomechanisms of the lesions and their clinical relevance in affected patients. (C) 2013 The Japanese Society of Child Neurology. Published by Elsevier B.V. All rights reserved.