In a previous study 30 asthmatic children with positive skin tests to cat dander, had bronchial challenge tests with histamine and antigen, quantitative skin tests with cat dander, and measurements of serum-specific IgE concentrations (first study). When bronchial responsiveness (BR) was found to be changed in the course of a follow-up period, these patients (4 had a decrease and 4 an increase in BR), as well as 2 patients with an unchanged BR, were restudied (second study). An increase in BR was associated with a change from a negative to a positive reaction to the inhalation of allergen. The patients in whom BR decreased, showed a less severe inhalation reaction to allergen in the second than in the first study. The inhalation reaction was not changed in the patients with an unchanged BR. The variation of allergen-induced bronchial obstruction could not be related to a change in sensitization to the allergen or in the baseline lung function, and is, thus, probably associated with the variation in BR.