A study was performed of EEG-magnetic resonance imaging (MRI) abnormalities in 7 Lennox-Gastaut syndrome (LGS) children and 3 epilepsia partialis continua (EPC) children with intractable generalized and partial motor seizures, respectively. In 4 children with LGS and 3 children with EPC, depth electrodes were implanted in the centromedian thalamic nuclei (CM) for seizure control. In all children with LGS, hyperdense, T2-weighted MRI signals were observed at the mesencephalic level of the brainstem, whereas none of the 3 children with EPC had this finding. Patients with idiopathic LGS without cerebral hemisphere MRI abnormalities showed generalized bilateral and symmetrical spike-wave EEG activity. Patients with symptomatic LGS with unilateral hemispheric MRI abnormalities demonstrated asymmetrical EEG activity with higher amplitude spike-and-wave over the normal hemisphere. Patients with EPC with unilateral hemispheric lesions had lateralized higher amplitude spike-wave over the damaged hemisphere. These data suggest that abnormal mesencephalic MRIs are a sign of bad prognosis in patients with LGS but not with EPC. Maximal amplitude paroxysmal EEG activities may indicate the abnormal hemisphere in patients with EPC or the normal hemisphere in those with LGS.