Overdrainage of the cerebrospinal fluid (CSF) and collapse of the ventricles, slit ventricles (SLV), can cause clinical symptoms and result in the slit ventricle syndrome (SLVS). The EEG changes and the frequency and type of epilepsy in patients with SLV was analysed from a material of 113 shunt-treated hydrocephalic children. During the follow-up time (mean 8.9 years), 63 patients (56%) had developed SLV. The age at initial shunting was significantly lower (1.2 years) in patients who developed SLV than for those who did not (2.7 years). After initial shunting generalized spike and sharp wave activity (SWA) developed more frequently in patients who developed SLV (81%) than in those who did not (54%). Severe generalized SWA developed almost entirely in patients in the SLV group. This severe generalized SWA disappeared from the EEG in patients after treatment of the SLVS. Epileptic seizures appeared after initial shunting in 44% of patients in the SLV group but in only 6% of the non-SLV group. Treatment of the SLVS decreased the frequency of epilepsy to a level corresponding with the non-SLV group. Repeated EEG evaluation of shunt-treated hydrocephalic children is a valuable aid in follow-up. If EEG abnormality appears after initial shunting, especially SWA, shunt malfunction and overdrainage of the CSF should be suspected.