ABSTRACT.Although the Lennox-Gastaut syndrome can be generally defined as consisting of clinical seizures of different types, slow spike-and-wave complexes on the EEG, and mental retardation, the prognosis of the syndrome has been shown to depend on many variables. Most cases of Lennox-Gastaut syndrome have their onset in childhood, with nearly one-quarter developing from West syndrome. Patients with later onset of the syndrome appear to have less severe mental retardation and seizure disorders than those with early onset. Status epilepticus occurs frequently in patients with Lennox-Gastaut syndrome, and can lead to mental deterioration or death. Effective anticonvulsant therapy seems to reduce the severity of the seizures and the mental retardation. Younger patients tend to have primary generalized seizures of many types, whereas older patients have a higher incidence of complex partial seizures. This progression is reflected in the EEG abnormalities, with younger patients having slow spike-and-wave and older patients having focal abnormalities.