We report the case of a girl with hemimegalencephaly (HME) and Lennox-Gastaut syndrome (LGS) treated by callosotomy at 1 year of age. Over 10 years, her seizure frequency and severity decreased markedly. Hemispherectomy is the main surgical option for HME although HME appears to correlate with a less favorable seizure outcome. However, the clinical presentation of LGS and possible generalized cortical dysplasia, which is indicative of a secondarily generalized epilepsy, might predict a favorable surgical outcome of corpus callosotomy in patients of HME, as in our case.