A 9-year-old boy with mental retardation and partial complex seizures is referred for magnetic resonance imaging. The MRI reveals increased signal intensity in the left hippocampus and mild atrophy of the left mammilary body. These findings are pathognomonic for mesial temoral sclerosis (MTS). MTS is a frequent cause of epilepsy in children and adolescents, and is clinically characterized by partial complex seizures, sometimes with secondary generalization. The diagnosis is made on the basis of the clinical examination and the typical MRI findings.