The study aimed to identify behavioural and cognitive characteristics in 29 children with Smith-Magenis syndrome. Cognitive assessments were undertaken on the children, and detailed interviews assessing sleep patterns, maladaptive behaviours, self-injury, hyperactivity and autism were carried out with their parents and teachers. The study identified high levels of sleep problems, aggression, self-injury, distractibility and autism in the sample, in comparison with rates reported for other groups of children with learning disabilities. These were associated with high levels of stress in the parents. It is concluded that the combination of difficulties and abilities identified in the present sample of children with SMS is indicative of a behavioural phenotype for the syndrome, and that there is an urgent need for intervention studies on the challenging behaviours posed by this group of children.