The autistic spectrum/pervasive developmental disorders (termed autism here) are neurobehavioral conditions which are characterized by severe qualitative impairments in social reciprocity and communication, associated with stereotypical behaviors and interests. Autism is frequently combined with other symptoms of neurological dysfunction, including abnormal electroencephalograms (EEGs) and clinical seizures in at least 50% of affected individuals. A subgroup of children who develop the impairments of autism after initially normal development (termed ‘autistic regression’) is of particular interest. These children have an increased incidence of abnormal EEGs similar to those found in individuals with Landau–Kleffner syndrome (LKS) (acquired epileptiform aphasia). Recognition of overlapping clinical and electrophysiological features in these two conditions has led to speculation that there may be a pathophysiological relationship between them. This commentary will discuss the diagnosis and treatment of autistic regression (AR) and LKS, to provide a clinical perspective on their sometimes confusing relationships.