Sixteen patients with asthma were challenged with solutions of Dermatophagoides pteronyssinus extracts. Fifteen patients demonstrated an early asthmatic reaction, and 10 patients also demonstrated a late asthmatic reaction, as demonstrated by a 15% decrease of FEV1 within 1 hour or between 3 and 8 hours after challenge, respectively. Airway responsiveness to inhaled methacholine, expressed as the dose causing a fall of FEV1 of 15% (Pd15), and the serum IgE level (RAST), expressed as percent of the total radioactivity added, were also measured. Multiple regression analysis of data demonstrated that both early asthmatic reaction, expressed as the allergen Pd15, and late asthmatic reaction, expressed as the FEV1 maximum fall from control values, can be predicted with similar accuracy (explained variance of 63% and 68%, respectively) by a model that incorporates simultaneously methacholine Pd15, percent of total radioactivity added by RAST, and baseline values of FEV1. We conclude that the late asthmatic reaction is more likely to occur and is more severe in patients with more severe underlying disease.