Hemimegalencephaly is a rare congenital anomaly of the brain characterized by unilateral hypertrophy, usually with an abnormal gyri pattern and derangement of the cortical architecture. We report 2 patients with hemimegalencephaly who presented with early-onset seizures, hemiparesis and developmental delay. An electroencephalogram showed hemispheric continuous spikes and spike-and-waves in one patient and repetitive spike-and-waves in the other. Magnetic resonance imaging showed left hemimegalencephaly in both cases and in case 1 pachygyria and heterotopia. A functional hemispherectomy was done on case 1 at the age of 33 months and on case 2 at 7 months due to difficulty in seizure control with antiepileptic drugs. The frequency of seizure dramatically decreased and there was an obvious improvement in neurologic development after surgical intervention. Pathology revealed disorganized lamination of the cortical layers with increased neuron size and bizarre-shaped neurons in both cases. Heterotopia of neurons and glia in the subarachnoid space was noted in case 1. Hemispherectomy should be performed as soon as possible when medical treatment fails to control seizures.