Despite the difficulty of defining behaviour disorder, most previous studies have reported a high rate of behaviour disorders in people with intellectual disability (ID). The aim of the present study was to establish the overall rate and types of behaviour disorders in a population-based sample of adults with ID. The other aim was to explore the possible risk factors that are associated with the overall rate as well as different types of behaviour disorders. One hundred and one adults with ID aged between 16 and 64 years were randomly selected from a sample of 246 such adults, i.e. those who were known to the Vale of Glamorgan Social Services Department in South Wales, UK. Thirteen behaviour disorders were rated according to the Disability Assessment Schedule. Background data on subjects were also collected, and were subsequently analysed to assess the relationship between different risk factors and behaviour disorders. Sixty-one subjects (60.4%) had at least one behaviour disorder of any severity or frequency. Twenty-three per cent of subjects showed aggression, 24% self-injurious behaviour, 36% temper tantrum, 26% overactivity, 29% screaming, 38% attention-seeking behaviour, 20% objectionable habits, 18% night-time disturbance and 12% of subjects showed destructiveness. Statistically significant associations were seen between the rate of overall behaviour disorder and the use of psychotropic medication, and between family and group home residence. The rate of aggression was significantly associated with the use of psychotropic medication. The rate of self-injurious behaviour was significantly associated with the severity of ID, female gender and poor communication abilities. The rate of temper tantrum was significantly associated with the use of psychotropic medication. Twenty-four subjects showed severe or frequent aggression, destructiveness, self-injury or temper tantrum, and 11 individuals showed real challenging behaviours. Severe behaviour problems were significantly associated with female gender, severity of ID, the presence of a history of epilepsy and attendance at day activities.