Background: Studies of people with intellectual disability suggest that several individual characteristics and environmental factors are associated with behaviour disorder. To date there are few studies looking at risk factors within specific syndromes and the relationship between early risk markers and later behaviour disorder. The key aim of the study was to determine if childhood risk factors were predictors of severe behaviour disorder in young adults with Down syndrome. Materials and methods: A longitudinal cohort study of 50 people with Down syndrome was assessed with the same behavioural measure used in both childhood and adult life. The early risk factors which were assessed included childhood functioning, childhood psychopathology and early family environment as determined by measures of parental mental health, quality of parental marriage and social background. Results: Childhood psychopathology and functioning were associated with severe behaviour disorder in adult life whereas childhood family environment was not. Conclusions: Children with Down syndrome at risk for severe behaviour disorder in adult life may be identified in childhood and appropriate interventions offered to reduce their risk.