Abstract 1. 1. The EEG was analyzed in seventeen children with non tumor hydrocephalus in the absence of clinical and/or radiological signs suggestive of involvement of the cerebral cortex. Only one case had convulsive seizures. Three had positive histories of obstetrical trauma, fetal involvement and meningoencephalitis respectively. 2. 2. In sixteen cases abnormalities were found. These consisted of: ( a) asymmetrical, polymorphic delta (9 cases); ( b) epileptiform activity (5 cases); ( c) generalized abnormalities (4 cases). 3. 3. Some of these abnormalities suggest a (subclinical) involvement of the cerebral cortex, even in cases of congenital hydrocephalus. 4. 4. EEG differences from cases of tumor hydrocephalus, and some of the factors determining these differences, are discussed.