BACKGROUND: There is a high risk that young children who show early signs of mental health problems develop symptoms in the same or overlapping areas some years later. The Strengths and Difficulties Questionnaire (SDQ) is widely used to screen externalizing and internalizing problems early in life. In Sweden 80-90% of all children aged 1-5 years go to preschool and preschool is thus an appropriate context for finding early signs of mental health problems among children. METHODS: This study is part of a longitudinal project too investigate the frequency of emotional and behavioural problems for children between 1 and 5 years of age in Sweden. The SDQ including the impairment supplement questions were rated by preschool teachers too establish Swedish norms for SDQ in preschool children. RESULTS: The sample involved 815 children with a mean age of 42 months (SD = 16, range 13-71 months). 195 children were followed longitudinally for three years. There were significant differences between boys and girls on all subscales except for the Emotional subscale. The prevalence of behavioural problems was similar to other that in European countries, except for Prosocial behaviour, which was rated lower, and Conduct problems, rated higher. Swedish children were estimated to have more problems in the preschool setting, scored by preschool teachers. The development of behaviour over time differed for the different subscales of SDQ. CONCLUSIONS: The teacher version of the SDQ, for 2-4 year-olds, can be used as a screening instrument to identify early signs of emotional distress/behavioural problems in young children. Preschool teachers seem to be able to identify children with problematic behaviour with the use of SDQ at an early age. The development of behaviour over time differs for the different subscales of SDQ. The Swedish norms for SDQ are to a large extent, similar to findings from other European countries.