Spectral and historical topographic mapping of EEG was done on 2 siblings with Landau-Kleffner syndrome. The clinical features of the elder sister were acquired aphasia developed at the age of 5, followed by convulsions a year later, and those of the younger brother were progressive ataxia, hemiparesis, urinary incontinence and convulsions at the age of 4 years and 10 months, followed by acquired aphasia a year later. The most prominent spectral mapping features were high spectral powers of delta, theta and alpha waves over the fronto-centro-parietal area. The power of alpha and beta wave bands, reflecting sharp or spike waves, varied spatio-temporally over the central, parietal, temporal and frontal areas. The historical mapping revealed variabilities of paroxysmal discharges in modes of propagation. These results suggested that electrophysiological dysfunction of the fronto-centro-parietal areas associated with markedly unstable paroxysmal discharges is the main feature of Landau-Kleffner syndrome.