BACKGROUND: The association between exacerbations of asthma and weather or air pollution is not well understood. The relationships between visits to the emergency room for asthma attacks and the meteorological, aerobiological, and chemical characteristics of the outdoor air have been evaluated. METHODS: The number of daily attendances for asthma attacks at the emergency room of Oulu University Central Hospital was recorded over one year together with daily meteorological readings (temperature, humidity, barometric pressure, rainfall), levels of air pollutants (nitrogen dioxide (NO2), sulphur dioxide (SO2), hydrogen sulphide (H2S), total suspended particles (TSP)), and pollen counts (birch, alder, pine, willow, total pollen). The relationship between the number of attendances and the measured variables was then analysed by multiple regression and stepwise discriminant analysis. RESULTS: The total number of attendances during the year was 232, with lower figures in summer and higher in winter. No association was found between visits for asthma attacks and airborne pollen levels or meteorological factors except for temperature, which had a low inverse correlation with attendance. The most significant correlations were found between asthma visits and the levels of NO2; those for SO2, TSP, and H2S were also significant. Intercorrelations between SO2 and temperature or NO2 and between temperature and TSP or NO2 were also found, but only NO2 correlated significantly with attendances after standardisation for temperature. CONCLUSIONS: Increased levels of pollutants, especially NO2, were associated with attacks of asthma, but the explanation for this is unclear. Air pollen levels were not associated with asthma attacks and only temperature among the meteorological factors had a small association with asthma.