Several studies on the prognosis of isocyanate-induced asthma show that a significant proportion of patients continue to experience asthmatic symptoms and nonspecific bronchial hyperresponsiveness after cessation of work, and that further exposure to isocyanates in sensitized subjects leads almost invariably to persistence of respiratory symptoms and of bronchial hyperresponsiveness and the deterioration of airway function. Specific bronchial reactivity to isocyanates may change after cessation of work; however, some subjects continue to be sensitive to TDI several months after cessation of work. The determinants of an unfavourable prognosis for asthma seem to be the same as those for other types of occupational asthma due to low molecular weight compounds (i.e. red cedar asthma): long duration of exposure before the onset of asthma, long duration of symptoms before diagnosis, airway obstruction, and dual airway response after specific challenge tests. Also, single acute exposure to high levels of TDI in the workplace (spills) can result in persistent nonspecific bronchial hyperresponsiveness. Potential mechanisms of persistence of symptoms and of nonspecific bronchial hyperresponsiveness may be chronic inflammation, bronchial smooth muscle alteration, autonomic nervous system disregulation.