Twenty-four patients with asthma of suspected allergic origin were subjected to a total of twenty-five bronchial provocations with the relevant allergen. Eighteen of them reacted with bronchial obstruction: eleven with an isolated early reaction, five with a dual reaction and two with only a late reaction. Extensive lung function tests were performed. The patterns of early and late reactions were qualitatively very similar regarding variables of pulmonary physiological function, but there were quantitative differences. Patients with dual reactions showed significantly greater changes in FEV1, RV and TGV during the early response than those with isolated early reactions. Maximum mid-expiratory flow rates with the patients breathing air and a mixture of He and O2 were measured before and 6 hr after the challenges. Four of five patients with both an immediate and a late response showed a decrease in the response to helium during the latter phase, suggesting increasing involvement of the small airways. An increase in the slope of the alveolar plateau (phase III) was observed in four of the five patients with dual responses and all five had increased CC%. It was concluded that the changes in pulmonary function occurring during the dual responses were widespread, involving the airways generally and resembling those in the clinically more severe asthma. Usually, individuals with dual reactions showed stronger reactions. In two cases, however, a late reaction was obtained without any preceding early reaction.