Early-onset hydrocephalus was identified as the common denominator in 5 of 12 consecutive patients with Landau-Kleffner syndrome or continuous spike wave in slow-wave sleep who presented during the last 6 years. This association seems to be more common than expected, and the course of epileptic disease and outcome differs from that of previous reports. We present the clinical cases and discuss the possible connection between early hydrocephalus and epilepsy, especially continuous spike wave in slow-wave sleep. A retrospective analysis was undertaken of the clinical course and electroencephalographic (EEG) data before and after the development of continuous spike wave in slow-wave sleep in a group of five children with early-onset hydrocephalus identified in three epilepsy clinics in Israel. Landau-Kleffner syndrome or continuous spike wave in slow-wave sleep was identified in 12 patients during 6 years (1995-2000). Five patients had a history of early-onset hydrocephalus. Three patients had ventriculoperitoneal shunts; two of them were never shunted. Patients' seizures and continuous spike wave in slow-wave sleep epileptiform activity was resistant to appropriate anticonvulsant treatment. Pediatric neurologists and neurosurgeons should be aware of the association between continuous spike wave in slow-wave sleep and early-onset hydrocephalus. When a child with hydrocephalus presents with behavioral, cognitive, or motor deterioration, he or she should undergo sleep EEG in addition to investigation of shunt function. Our series demonstrated the presence of continuous spike wave in slow-wave sleep and localization-related epilepsy that did not originate from the shunt site. This group of patients is heterogeneous, and the exact trigger for continuous spike wave in slow-wave sleep development is still unclear.