A number of factors are associated with the development of childhood asthma. The purpose of this study was to establish the prevalence of childhood asthma and to explore the socioeconomic background factors associated with childhood asthma in a Norwegian cohort of children aged 4-5 yrs.A questionnaire was given to parents in connection with the ordinary child control of 4-5-yr-old children in Vestfold county, Norway. In addition to the question "Has the child at present or ever had asthma?", a number of medical and socioeconomical background factors were registered.Of the 2,430 parents, 1,913 (79%) responded. Of the 163 (cumulative prevalence 8.7%) children with confirmed asthma, 19 did not use any medication and were regarded as having outgrown their asthma. Several background factors were significantly associated with asthma in a logistic regression analysis: few rooms at home, psychosocial problems, fever more than three times during the last year, hay fever, reaction to food and mother or father with chronic disease.The findings indicate that socioeconomic background factors are associated with asthma in childhood, in addition to other known risk factors.