Angelman syndrome is a neurogenetic disorder that is characterized by impairments in neurological, motor, and intellectual functioning. This study compared 27 participants with Angelman syndrome to clinical and community participants with developmental disabilities of mixed etiology to determine whether Angelman syndrome is associated with a distinctive pattern of behavioral functioning. The groups with and without Angelman syndrome were matched on chronological age, gender, and level of intellectual functioning. The dependent measure was parent ratings of maladaptive behavior using the Aberrant Behavior Checklist. The Angelman syndrome group had significantly lower scores on measures of irritability and lethargy. Results contribute to the delineation of a behavior phenotype for the syndrome. Reprinted by permission of the publisher.