We studied the clinical and EEG-findings in 28 adult patients (aged 20-53 years) with Angelman syndrome (AS). Twenty-three showed a maternal chromosome 15q11-13 deletion; in 5, the diagnosis was based on a combination of typical clinical findings. Compared to the clinical manifestations present in childhood, "coarsening" of facial traits (100%), thoracic scoliosis (71%), and being wheelchair-bound (39%) were found more frequently. Paroxysms of laughter were still observed in adulthood (79%), but less frequently than in childhood. Most adult patients could feed themselves, but needed help with many daily activities. The majority (82%) had epileptic seizures. Abnormal EEG-activity consisting of 2-3/s rhythmic triphasic waves of high amplitude with a maximum over the frontal regions, which has been identified in many AS children, was found in 67% of these adult patients.