This paper reviews the known respiratory effects of isocyanates. There is good evidence to indicate that isocyanates: cause chemical bronchitis/pneumonitis; are potent pulmonary sensitizers capable of causing "isocyanate asthma"; cause nonspecific airways disease, including chronic bronchitis; can induce a general asthmatic state; and can cause hypersensitivity pneumonitis. Similar dose-response relationships are seen for both acute and chronic effects. There are plants operating in which exposures are well controlled and in which no respiratory effects can be detected. Suggestions are provided for preplacement assessment and periodic surveillance for workers exposed to these compounds.