We report a 6-year-old girl with diffuse subcortical heterotopia (band heterotopia), an unusual type of ectopic gray matter on magnetic resonance imaging (MRI). Her cerebral parenchyma had four layers consisting of cortex, thin outer white matter, diffuse subcortical heterotopia, and inner white matter around the lateral ventricles, giving the appearance of a 'double cortex'. The overlying cortex had midly broad gyri, especially in the frontal lobes. MRI showed the appearance of laminar heterotopia, a classical pathological entity. The double cortex syndrome is presumably a radiological delineation of laminar heterotopia. Her development was slightly delayed and she was clumsy and easily upset, with poor impulse control. However, her daily life was largely unaffected in spite of the severe abnormalities on MRI. She had abnormal movements during sleep, and sleeping EEG showed high voltage spindles in the bilateral frontal areas but no epileptic discharges. It could not be determined whether or not these movements were epileptic. The double cortex syndrome, an anomaly of the central nervous system associated with mental retardation, epilepsy, behavioral problems, and exclusive occurrence in girls, will be established as a clinico-radiological entity.