Studies in Britain and the US indicate that 10-15% of people with mental retardation show challenging behaviour, like attacking others (aggression), self-injurious behaviour, destruction, and other difficult, disruptive or socially unacceptable acts. Most researchers indicate that challenging behaviour is more common among adolescents and young adults, among males, is associated with autism, and increases with lack of communication skills and severity of mental retardation. These factors can be understood as risk markers, and some of them can be decreased by preventive and treatment interventions, at least in principle. The present study confirmed most of the previous findings, with some exceptions: the prevalence of more demanding challenging behaviour was somewhat lower in the present study, and no association between gender and challenging behaviour was found. We also concluded that declining prevalence of challenging behaviour at older ages is not a result of a young age structure of the population.