Background: In Sweden, 80-90% of children aged 1-5 years attend preschool, and that environment is well suited to identify behaviours that may be signs of mental health problems. The Strengths and Difficulties Questionnaire (SDQ) is a well-known short and structured instrument measuring child behaviours that indicate mental health problems well suited for preschool use. Aim: To investigate whether SDQ is a reliable and valid instrument for identifying behavioural problems in children aged 1-3 years and 4-5 years in a Swedish population, as rated by preschool teachers. Methods: Preschools situated in different sized municipalities in Sweden participated. The preschool teacher rated each individual child. Concurrent validity was tested using the Child-Teacher Report Form (C-TRF) and Child Engagement Questionnaire (CEQ). Exploratory factor analysis was conducted for age groups, 1-3 years and 4-5 years. Results: The preschool teachers considered most of the SDQ items relevant and possible to rate. For the children aged 1-3 years, the subscales 'Hyperactivity' (Cronbach alpha=0.84, split half=0.73) and 'Conduct' (Cronbach alpha=0.76, split half=0.80) were considered to be valid. For the age group 4-5 years, the whole original SDQ scale, 4-factor solution was used and showed reasonable validity (Cronbach alpha=0.83, split half=0.87). Conclusion: SDQ can be used in a preschool setting by preschool teachers as a valid instrument for identifying externalizing behavioural problems (hyperactivity and conduct problems) in young children. Clinical implications: SDQ could be used to identify preschool children at high-risk for mental health problems later in life. (author abstract)