Results of isocyanate challenge tests performed on 63 workers referred with a diagnosis of probable isocyanate asthma between 1974 and 1988 were reviewed. Thirty (48 percent) had an acute episode of asthma with a greater than 20 percent decline in FEV1 following subirritant exposure to isocyanates. No difference in the frequency or type of respiratory complaints between isocyanate reactors and nonreactors was found. No differences in lung function results were present when comparing smoking and ex-smoking reactors and nonreactors. In never-smokers with complaints consistent with isocyanate-induced asthma, the presence of obstructive lung disease increased the likelihood that isocyanate-induced asthma was present. Bronchial responsiveness to methacholine occurred in nearly all isocyanate reactors but predicted isocyanate-induced asthma in only 68 percent of the workers. In nearly all cases of challenge-confirmed toluene diisocyanate (TDI)-induced asthma, a 15-min exposure to 20 ppb of the commercial TDI mixture (80:20 2,4:2,6) provoked asthma. Conversely, in the absence of an asthmatic response following exposure to this dose for this duration, a second exposure at this concentration for a longer time would be reasonable to confirm the absence of isocyanate-induced asthma. Among workers employed in the production of polyurethane foam and confirmed to have TDI-induced asthma by inhalation challenge to the different TDI isomers, there appeared to be increased airway reactivity to the 2,6 isomer. This may have relevance to the frequency and intensity of respiratory symptoms that workers with TDI-induced asthma develop in differing industrial settings.