Hemimegalencephaly is a rare brain malformation characterized by cerebral asymmetry and cortical dysplasia. Infants with the condition present with early seizures and severe encephalopathy. Five patients were studied with computed tomography and magnetic resonance (MR) imaging. MR imaging was the most efficient diagnostic method for this rare entity. It demonstrated brain hemispheric hypertrophy with lateral ventricle dilatation, abnormal gyral pattern, and a thick cortex on the enlarged side. The images correlate well with the known pathologic data.