Our objective was to explore and compare, in the context of other exposures, lay and professional perceptions of the links between urban air pollution and children's asthma. We used a triangulated survey approach, using quantitative questionnaire surveys enriched by qualitative interviews. Derivation of indicators of actual local air quality used modelled air pollution and a geographical information system. Our setting involved families and community health professionals in the London borough of Ealing, and pediatric respiratory specialists across the United Kingdom. Participants included 863 parents of children aged 3-11 years, 151 reporting currently asthmatic children, of whom 20 were extensively interviewed; 98 local general practitioners and 50 practice nurses; and 75 paediatric respiratory consultants and 55 specialist nurses. Main outcome measures involved views about the links between urban air pollution and children's asthma, relative to other triggers. Comparison of assessments of local air quality, with actual pollution levels, was made by parents with and without asthmatic children. Many parents were unsure as to what factors initiate asthma, but the most frequently cited was traffic pollution; it was also considered important in the exacerbation of asthma. Health professionals' assessments were inconsistent: specialists conformed to the dominant literature dismissing strong links between air pollution and asthma, while local clinicians reflected the views of parents in their community. Surrounding parents' views were difficulties defining exposures to urban air pollution, underlying concerns about risks to general health, perceived lack of control, unclear expert opinion, and widely accepted informal "messages" which assumed strong links. Parents with experience of asthma were found to have significantly less accurate (negatively biased) perceptions of local air quality. In conclusion, reactions to uncertainty surrounding associations between asthma and urban air pollution varied: parents' concerns were heightened (and propagated by other influences), specialist clinicians were dismissive, and community clinicians fell between these extremes.