Thirty-five subjects with occupational asthma due to toluene diisocyanate (TDI) exposure were examined. All the subjects were studied with inhalation challenges with TDI and with methacholine. TDI asthma was documented by a positive inhalation challenge to low levels of TDI. Airway responsiveness to methacholine was in the range of asthmatic patients at the time of diagnosis. After an average follow-up interval of 10 months, all the subjects were re-examined. Of the 35 subjects examined, 30 subjects (85.7%) left the workplace, and 5 remained in the same job. Twenty-seven subjects (77.1%) continued to have asthmatic attacks requiring medication for relief of symptoms. At follow-up examination, TDI asthma was documented by a positive inhalation challenge to TDI in 27 subjects. Of these 27 TDI reactors, 22 subjects were removed from occupational exposure to TDI. The TDI reactors had persistent respiratory symptoms and airway hyperresponsiveness to methacholine. At follow-up visit, 8 subjects (22.9%) lost sensitization to TDI; 5 subjects (62.5%) in this group had also normal airway responsiveness to methacholine after removal from exposure. Only 1 subject among the TDI nonreactors complained of mild respiratory symptoms. At diagnosis, there were no significant differences between subjects who recovered and those who did not with regard to age, smoking habits, atopy, duration of exposure to isocyanates, duration of symptoms, baseline FEV1 (% pred), and baseline airway responsiveness to methacholine.(ABSTRACT TRUNCATED AT 250 WORDS)