The visually evoked potential was abnormal in 17 of 18 patients with Huntington disease. Both early and late wave components were affected, and the averaged amplitude for the patients was reduced in comparison with 50 normal control subjects. Despite striking attenuation and disorganization of the complex, latency of initial wave components was normal. The abnormality was not present in patients with a variety of other nonfocal cerebral disorders nor in 13 children of patients with Huntington disease.