Prolonged electroencephalographic frequency modulated radio telemetry and video-tape monitoring (TEEG-VR) is widely used as a diagnostic aid in the evaluation of patients with suspected epilepsy. Use of this technique with 38 severely to profoundly mentally retarded patients to differentiate epileptic from nonepileptic behavior was reviewed. For only 2 patients was the behavior in question not recorded. Although all patients studied had abnormal EEGs and 90 percent had histories of epileptic seizures, only 38 percent of the behavior in question recorded during TEEG-VR was epileptic seizures. Criteria used for nonretarded patients to differentiate epileptic seizures from nonepileptic behavior was not helpful for this group of patients.