Two hundred and fifty six attacks of acute bronchial asthma occurring in 169 children aged over 2 years were studied during a two year period. More attacks occurred during spring and autumn than at other times of the year. In 73 patients (29%) a respiratory virus infection was diagnosed, with the same seasonal variation as the asthmatic attacks. Most of the virus infections were caused by rhinovirus (45%) and respiratory syncytial virus (19%). There was no significant correlation between asthmatic attacks in patients with birch pollen, grass pollen, or Cladosporium herbarum allergy and counts of the respective pollen or spores in the air. More seasonal attacks occurred in patients with cladosporium allergy than in patients without cladosporium allergy but there was no seasonal variation among birch or grass pollen allergic patients. Information about exposure to animals was obtained in only 12% of attacks occurring in 121 patients with allergy to animal dander. The single precipitating factor most frequently associated with acute asthma was respiratory virus infection.