The mental health, adaptive behaviour and intellectual abilities of people with Down syndrome (n=129) were evaluated in a population-based survey of social and health care records. Females had better cognitive abilities and speech production compared with males. Males had more behavioural problems than females. Behaviour suggestive of attention deficit hyperactivity disorder was often seen in childhood. Depression was diagnosed mainly in adults with mild to moderate intellectual disability. Autistic behaviour was most common in individuals with profound intellectual disability. Elderly people often showed decline of adaptive behaviour associated with Alzheimer's disease. Case descriptions are presented to illustrate the multitude of mental health and behavioural issues seen from childhood to old age in this population. (Contains 6 tables.)