Background: There is sparse literature about problem behaviour in young children with Williams syndrome (WS) and little consideration of the perspectives of multiple respondents. Methods: Problem behaviour of 35 children with WS ages 2 to 6 was examined based on both parent and teacher report using the Achenbach preschool forms. Results: The most prominent areas of difficulty based on both parent and teacher report were attention problems, pervasive developmental problems and emotion reactivity difficulties. Some rater differences were observed; most notably, teachers reported more externalising behaviour problems including more aggressive behaviour, Attention Deficit/Hyperactivity problems and Oppositional Defiant problems than did parents. Few relations to intellectual functioning, age or gender were observed. Discussion: Some aspects of problem behaviour evident in older children (e.g. attention problems, social problems) are also apparent for young children with WS, while other areas are less prominent (e.g. anxiety). The implications of the findings for understanding the behavioural phenotype associated with WS are discussed.