Before and after administration of intravenous immunoglobulin (IVIg), cerebrospinal fluid (CSF) was examined in a homogeneous group of 15 patients with cryptogenic types of West syndrome (WS) and Lennox-Gastaut syndrome (LGS). The purpose of the present CSF study was: (i) to elucidate possible etiological factors and consequences of these severe forms of epilepsy, and (ii) to elucidate mechanisms of action and adverse effects of IVIg. Hypotheses concerning etiological factors like central nervous system infections, neuroimmunological disorders, or disturbances in neurotransmitter metabolites could not be confirmed. These normal CSF findings are in accordance with the concept of a cryptogenic etiology of the epilepsies in the reported patients. Nor could we confirm hypotheses concerning seizure consequences, such as increased blood-CSF permeability, increased markers of brain cell destruction, or increased metabolic components. Following IVIg administration in these patients, all with an on the whole undisturbed blood-CSF barrier permeability as measured by Q albumin, the CSF IgG concentrations increased significantly and proportionally to the Q albumin level. No signs of adverse effects of IVIg such as aseptic meningoencephalitis were found in 165 infusions of IVIg performed in the 15 children.