Angelman syndrome (AS) is a genetic disorder that is associated with a deletion on chromosome 15, and is characterized by abnormalities or impairments in neurological, motor and intellectual functioning. While behaviour problems have been reported in clients with AS, relatively little is known about their developmental course and outcome. In this study, data on the nature and prevalence of behaviour problems among clients with AS were gathered from two sources: (1) a review of published case reports; and (2) parent responses to a survey of behaviour problems in a small (n = 11) sample of children with AS. Data from both sources showed that behaviour problems were present in males and females of all ages, and included language deficits, excessive laughter, hyperactivity, short attention span, problems with eating and sleeping, aggression, noncompliance, mouthing of objects, tantrums, and repetitive and stereotyped behaviour. Identification and treatment of severe behaviour problems in clients with AS may improve their adaptive functioning.