In two sets of male homozygous twins with tuberous sclerosis, one twin in each set suffered frequent generalized seizures from early life, and the second either had no seizures or had only short-lived seizures. One twin of each pair is of normal intelligence, and the other is mentally subnormal. We propose that generalized seizures occurring in early life are an important mechanism in an apparently progressive dementing process in some infants and children with tuberous sclerosis. If our assumption is correct, an early attempt to control generalized seizures in these patients is indicated even in the presence of the cortical, subcortical, and subependymal pathologic changes characteristic of cerebral tuberous sclerosis.