Somatosensory evoked potentials (SEP) were recorded in ten patients with the Rett syndrome. Seven cases showed abnormal SEP, and five of these cases showed giant SEP. The five cases with giant SEP were relatively young girls with epilepsy and parieto-central spikes on the electro-encephalogram (EEG). Older patients did not show giant SEP. These giant SEP may be the reflection of cortical hyperexcitability and they may subside as the disease progresses.