Lennox-Gastaut syndrome (LGS) is a childhood epileptic encephalopathy characterized by an electroclinical triad of generalized slow spike wave (SSW) activity in the EEG, multiple types of epileptic seizures, and slow mental development. It is usually subdivided into symptomatic and cryptogenic types, the latter accounting for at least one fourth of all patients. Symptomatic cases are due to diverse cerebral conditions, which are usually bilateral, diffuse, or multifocal, involving cerebral gray matter. Twenty percent of all patients with LGS have prior infantile spasms with hypsarrythmia. The characteristic interictal EEG pattern of LGS is 1.5 to 2.5 Hz SSW activity, which is bilaterally synchronous, dominant over the frontocentral regions, and usually symmetric. There are varying degrees of slowing of the background. Sleep discloses paroxysms of generalized fast (10 to 25 Hz) rhythmic activity.