During recent years in Western countries, diisocyanates are one of the main causes of occupational asthma. The mechanism of diisocyanate-induced asthma is still unknown but recent evidence suggests immunological mechanisms, including cell-mediated immune responses. Immune responses to isocyanates may result in different illnesses, cell- and/or antibody-mediated entities. In addition, irritative, toxic, and mutagenic effects may occur. This review summarizes current knowledge of the pathomechanisms, including immunological and nonimmunological (mutagenic and genotoxic) aspects of isocyanate disorders.