In an add-on pilot study, a group of 15 children with cryptogenic and intractable West syndrome (3) and Lennox-Gastaut syndrome (12) received intravenous immunoglobulin (IVIg, 0.4 g/kg body weight per day for 5 consecutive days, followed by the same dose once every 2 weeks for 3 months). Five patients had been treated previously with ACTH without success. The reduction in clinical seizures averaged 70%. Electroencephalographic (EEG) recordings revealed a mean reduction in epileptic discharges of 40%. In all 15 patients, acceleration of EEG background activity occurred, and psychomotor development improved. Prior to IVIg administration, CSF examinations were normal. After IVIg administration, the serum total IgG concentration increased by an average of 76%, and the CSF IgG concentration by 44%. According to our data, IVIg crosses the blood-CSF barrier, and might be effective in the treatment of West syndrome and Lennox-Gastaut syndrome. We suggest it should be considered when other treatments, such as ACTH, have failed.