While general population studies indicate an increase in the rate of psychiatric disorder in adolescence, little is known about the course of mental health and behaviour problems between childhood and adolescence in young people with severe intellectual disabilities. From a sample of 111 children with severe intellectual disability who had been identified from the registers of six special schools at 4-11 years of age, 82 were traced and reassessed 5 years later at the age of 11-17 years. Behaviour problems were assessed by means of parental interviews conducted in the family home and parent and teacher questionnaires. Parental reports of psychiatric diagnoses were checked against health records. With most behaviour problems, including aggression, destructive behaviour and self-injury, there was little difference in rates between the two assessment occasions. However, in spite of this overall pattern of stability, the rates of some behaviour problems, including overactivity, showed significant reductions between childhood and early adolescence. Persistence rates for most behaviour problems appeared comparable to those reported for similar behaviours in general population studies of children. There was no significant difference in the proportion of cases with psychiatric diagnoses between the two assessment occasions, although brief psychotic episodes emerged in three cases in adolescence. The findings suggest that the prevalence of mental health and behavioural problems in young people with severe learning disabilities remains relatively stable between childhood and adolescence, although some specific behaviour problems diminish. However, a small minority of children may develop severe psychiatric disorders in adolescence.