A total population study of 4797 six-year-old children attending the public pre-schools in the city of Göteborg (Gothenburg) has been carried out. A questionnaire with 34 questions about MBD-related problems was distributed to all pre-school teachers. Three thousand four hundred and forty-eight questionnaires were completed. Factor analysis of the questionnaire and empirical results from a pilot study provided the basis for selecting for further study children with pre-school signs and symptoms suggestive of MBD. Neurological, psychiatric and psychological assessment of 82 children with, and 59 children without, pre-school symptoms of MBD revealed that in the index groups 41% of the children, and in the control group 3% of the children, had MBD. Extrapolation procedures gave a total population frequency of 1.2% with severe MBD and a further 5.9% with mild-moderate MBD. A very large questionnaire refusal rate (28%) is discussed. The relevance of the calculated frequency figures, especially as regards the mild-moderate MBD category, cannot be properly evaluated until long-term follow-up has been completed.