To determine the importance of airway inflammation for late asthmatic reactions induced by toluene diisocyanate (TDI), we investigated whether prednisone prevented them [corrected] by modifying the associated airway inflammatory reaction. We measured FEV1 before and at regular intervals after exposure to TDI and performed bronchoalveolar lavage at 8 hours after TDI in two groups of subjects with previously documented late asthmatic reactions, in one group, after no treatment, and in the other group, after treatment with prednisone (50 mg/day for 4 days). After no treatment, each subject developed a late asthmatic reaction, an increase in airway responsiveness, polymorphonuclear leukocytosis, and increased albumin in bronchoalveolar lavage. By contrast, after treatment with prednisone, no subject developed a late asthmatic reaction or an increase in airway responsiveness, and the number of leukocytes and the concentration of albumin were normal in bronchoalveolar lavage. These results suggest that late asthmatic reactions induced by TDI may be caused by airway inflammation and that prednisone may block them [corrected] by inhibiting the inflammatory reaction of the airway induced by TDI in sensitized subjects.