The most consistent electroencephalographic finding in 16 cases of Sturge-Weber syndrome was a unilateral reduction of background amplitude in the waking record. Comparable asymmetries were noted in those patients in whom sleep recording also was done. Physiologic responses (to hyperventilation and photic driving) usually were decreased on the involved side. These hemispheric electroencephalographic abnormalities are detectable in infancy even before the characteristic intracranial calification develops. Epileptiform activity, when focal, was limited to the involved hemisphere.