An EEG study has been carried out on 19 children (including siblings in 3 families) with clinical features of Angelman syndrome. The age at time of the first EEG ranged from 11 months to 11 years with the majority under 5 years. Six children had no history of seizures at the time of the first EEG. One or more of the following EEG abnormalities were seen in all patients: 1. Persistent rhythmic 4-6/s activities reaching more than 200 microV not associated with drowsiness. 2. Prolonged runs of rhythmic 2-3/s activity (200-500 microV) often more prominent anteriorly, sometimes associated with discharges (ill-defined spike/wave complexes). 3. Spikes mixed with 3-4/s components usually more than 200 microV mainly posteriorly and facilitated by, or only seen with, eye closure. Two and sometimes three of these EEG features could be present in the same record particularly at a young age. The appearance of discharges mixed with slow components on eye closure was the commonest finding seen at some stage in 17 patients (aged from 11 months to over 12 years). The EEG features of Angelman syndrome appear to be sufficiently characteristic to help identify patients at an early age before the clinical features become obvious and at a time when genetic counselling may be particularly important.