BACKGROUND: The high and increasing prevalence of childhood asthma is a major public health issue. Various risk factors have been proposed in local studies with different designs. METHODS: We have made a questionnaire study of the prevalence of childhood asthma, potential risk factors and their relations in four regions in Scandinavia (Umeå and Malmö in Sweden, Kuopio in eastern Finland and Oslo, Norway). One urban and one less urbanized area were selected in each region, and a study group of 15962 children aged 6-12 years was recruited. RESULTS: The prevalence of symptoms suggestive of asthma varied considerably between different areas (dry cough 8-19%, asthma attacks 4-8%, physician-diagnosed asthma 4-9%), as did the potential risk factors. Urban residency was generally not a risk factor. However, dry cough was common in the most traffic polluted area. Exposure to some of the risk factors. such as smoking indoors and moisture stains or moulds at home during the first 2 years of life, resulted in an increased risk. However, current exposure was associated with odds ratios less than one. CONCLUSIONS: Our findings were probably due to a combination of early impact and later avoidance of these risk factors. The effects of some risk factors were found to differ significantly between regions. No overall pattern between air pollution and asthma was seen, but air pollution differed less than expected between the areas.