Changes over time in the pattern of behaviour problems shown by 91 children with Down's syndrome, together with maternal appraisal of the seriousness of reported problems, were investigated through questionnaires completed by mothers. Fiftyfour individual behaviours were studied at mean ages 9 years 2 months and 13 years 9 months. There was a significant decline in the overall frequency of behaviour problems and in some disturbed behaviours, sleeping and toileting problems, and over-activity. Only one problem-lying-became more common. However, many problems were found to be persistent among the same children.While mothers viewed relatively few behaviours as definite problems, a minority faced a multiplicity of perceived problems at both points in time. Running away was a common problem which particularly concerned mothers. Sleeping problems were generally seen as less problematic by Time 2, while stealing had become more so. Some individual problems were associated with the gender, age and developmental level of the children. Results suggest that behaviour problems may become set relatively early in life, and that families may benefit from help in preventing and dealing with problems well before the teenage years.