A specific IgE-mediated response was evaluated in twenty-eight workers exposed to TDI or MDI, with diagnosis of occupational asthma and positive to bronchial provocative challenge. The presence of anti-diisocyanate IgE was observed in 27% of subjects exposed to TDI and 83% of those exposed to MDI, particularly in individuals who experienced an acute massive exposure. An immediate-type response to bronchial provocative test was found in 66% of individuals with specific antibodies. Specific IgE are prevalent (91%) in subjects who developed symptoms before 6 years of exposure to isocyanates. The results suggest an association between the presence of specific IgE, early asthmatic symptoms and heavy episodic exposure.