Occupational asthma accounts for a significant percentage of all asthma. Prevalence varies markedly, depending on host factors, the agent, and the manufacturing facility. The number of agents capable of inducing occupational asthma is large and will continue to increase as new agents are introduced into the workplace. Once an individual is sensitized, very low concentrations of the agent can initiate an asthmatic response. In many cases the mechanisms of the asthma are yet to be determined. The ultimate outcome of both the specific asthmatic response and the nonspecific bronchial hyperreactivity that usually occurs is variable, but in some cases it can continue for decades. Future needs are a better understanding of the prevalence of occupational asthma and the causative mechanisms involved in the disease and prophylactic or "desensitizing" measures. Environmental experts should explore methods to decrease the exposure of workers, and manufacturers should also begin to explore the possibility that chemical modification of their products may reduce their asthmatic potential (e.g., MDI, which has a vapor pressure 1000 times less than TDI, appears to be a significantly less potent inducer of occupational asthma). Finally, the physician, by identifying individuals with occupational asthma, will help improve our understanding of the disease, the mechanisms involved, and the development of therapeutic and preventive modalities.