The dysembryoplastic neuroepithelial tumor (DNT) is a rare, recently recognized neoplasm, characterized by a mixed glial-neuronal cell proliferation. We studied 11 cases of DNT including 6 males and 5 females (age range 2.1-39.4 years, mean 13.8 years). All patients presented with chronic epilepsy of 4.8 years mean duration. Nine tumors were located in the temporal lobe and 2 in the frontal lobe. Four tumors were located on the right side and 7 on the left. All tumors were characterized by a predominant oligodendrocyte-like cell component with intermixed normal appearing neurons and astrocytes. All were focally microcystic and 7 of 9 evaluable tumors were multinodular. Other observed histologic features included a focal arcuate vascular pattern in 7 tumors, calcification in 4 tumors, and a rare mitotic figure in 2 tumors. Necrosis and vascular hyperplasia were not observed in any of the tumors. Cortical architectural disorganization (cortical dysplasia) was observed in 9 of 10 evaluable cases. MIB1 (a marker of cellular proliferation) immunostaining was performed in all 11 tumors and MIB1 indices (number of positive tumor cells/100 tumor cells counted) ranged from 0-0.6 (mean 0.2). Four patients required at least 1 additional surgical procedure for tumor recurrence, related to an incomplete initial excision 2.1-4.4 years after their initial operation. All patients are seizure-free at their last follow-up. DNTs are low grade neoplasms which typically present with chronic epilepsy, are most frequently located in the temporal lobe, and occur most frequently in pediatric patients. Their appearance and association with cortical dysplasia suggest a maldevelopmental origin. The generally slow growth and relatively benign nature of DNT is reflected by their low MIB1 index.