Changes in airway responsiveness to histamine after allergen inhalation challenge were studied in 14 nonsmoking atopic adult subjects with asthma. Inhalation challenges with allergen and with phosphate-buffered saline (control challenge) were performed single blind in random order, with an interval of 14 days. The development of a late asthmatic response was accompanied by an increase in airway histamine responsiveness that was significant when it was compared with the airway histamine responsiveness after the control challenge at 3 hours (p less than 0.01), 24 hours (p less than 0.01), and 48 hours (p less than 0.02), with recovery at 2 weeks after allergen inhalation. The 3-hour changes in airway responsiveness occurred independently of changes in airway caliber and correlated with the magnitude of the subsequent late response (r = 0.86; p less than 0.001). These results suggest that the tissue events (possibly airway inflammation) that underlie the late asthmatic response may occur before this response becomes clinically apparent.