The ketogenic diet is a therapeutic diet used to treat medically refractory epilepsy in children. It was found to be effective and safe. Apart from a reduced number of seizures, positive cognitive effects were described. The mechanisms of action are not fully understood, but both antiseizure and antiepileptogenic effects were proposed. Among other changes ascribed to the introduction of the diet, changes in electroencephalogram patterns might contribute to an understanding of the effects of the ketogenic diet. In this study, 23 children (mean age, 6.5 years) with pharmacoresistant epilepsy were started on the diet. They were examined via 24-hour ambulatory electroencephalogram directly before starting the diet, and after 3 months of treatment. The changing electroencephalogram pattern was evaluated qualitatively and semiquantitatively. Background activity, interictal epileptiform activity, ictal activity, and seizure reduction were evaluated. Quality of life was estimated on a visual analog scale. In 15 of 23 patients, the electroencephalogram indicated improvement in terms of more normal background activity or decreased interictal epileptiform activity. This improvement was seen in both seizure-reduction responders and nonresponders, and was not predictive of response to treatment.