Findings of virus infection were prospectively studied in 133 episodes of obstructive respiratory difficulty in 90 children aged 2 to 15 years. A viral infection was confirmed in 16 episodes (12%). In a further 11% of cases serological evidence of a recent viral infection was found. Respiratory syncytial virus (RSV) accounted for half of the positive viral results. Skin prick tests were performed after the second episode of bronchial obstruction. Of the 79 children tested, 57% had at least one positive result to house dust mite allergen and/or to one of the four seasonal allergens studied. Reactivity to birch pollen was most common. There was no significant correlation between the frequency of episodes in children with birch, timothy grass and mugwort pollens or with Cladosporium herbarum allergies and counts of the respective pollens or spores in the air. Blood eosinophilia was found in 34% of cases immediately after the episodes, but no seasonal variation occurred. It was impossible to differentiate the episodes initiated by viruses from those initiated by seasonal allergens. Thus exposure to seasonal allergens is a less common precipitant of respiratory difficulties from bronchial obstruction than are virus infections.