Bronchial asthma is very likely caused by the interaction of genetic susceptibility with an adverse environment. The mechanisms involved in asthma induced by toluene diisocyanate (TDI), a low molecular weight compound, are still unknown, but increasing interest has been expressed in the genetic component of this disease. In two collaborative studies on the role of genetic factors in isocyanate-induced asthma, a significant positive association with HLA class II DQB1*0503 allele and a negative association with HLA class II DQB1*0501 were observed. In the second study we also observed a single amino acid difference at position 57, with Aspartic Acid in DQB1*0503 allele and Valine in DQB1 allele, and we concluded that the presence of a DQB1 allele with the charged residue Aspartic Acid at position 57 has a role in creating susceptibility for isocyanate-induced asthma. These findings suggest a direct involvement of HLA class II antigens in the pathogenesis of isocyanate-induced asthma. Efforts must continue to improve the possibility of early diagnosis of isocyanate-induced asthma, and at the same time the social and ethical implications of a genetic screening at the workplace must be taken into consideration.