One-hundred and twenty-four mentally retarded persons with behaviors suggestive of epilepsy were monitored with an 8-channel radiotelemetered electroencephalograph-video recording system (TEEG-VR). Target behaviors were identified by the clinical description of the primary care providers before the TEEG-VR sessions. Whenever possible, the known antecedents of those behaviors were replicated. The target behaviors were considered epileptic when they were observed simultaneously with epileptiform EEG patterns, and pseudoepileptic when nonepileptiform patterns were observed. Twenty persons were classified as epileptic, 50 as pseudoepileptic, 11 as both epileptic and pseudoepileptic, and 43 as inconclusive. Among the pseudoepileptics there were 15 with abnormal EEGs and 4 with epileptiform EEGs. The most frequent topographies of behavior were not significantly related to diagnosis. These included myoclonus, eye blink, head drop, cessation of ongoing activity, and hand and arm automatisms. The diagnosis of epilepsy in mentally retarded persons, on the basis of clinical description, interictal EEG, and medical history, may be inaccurate. TEEG-VR is extremely useful for obtaining a definitive diagnosis of each target behavior.