The relations between epileptogenic focus seated outside the temporal lobe and childhood epileptic encephalopathies with diffuse slow spike-waves were studied. Eleven patients (3 with clinical and electroencephalographic evidences of cortico-mesial lesions and 8 with evidences of diffuse lesions in one of the hemispheres) were selected among 32 patients suffering from Lennox syndrome. Clinical history, findings of neurological examination and focal electroencephalographic abnormalities are important factors for ascertaining the localization of the primary cerebral lesion. No physiopathogenic datum permits to exclude from the Lennox syndrome the sequels of myoclonic encephalopathy with hypsarrhythmia or the cases with clinical and electroencephalographic signs of cortical or hemispheric localization when secondary bi-synchronous electroencephalographic discharges are present and the characteristic clinical symptoms and response to therapy are observed.