With the accelerating pace of new knowledge about occupational asthma, systematic epidemiological approaches have yielded important new insights into its clinical characteristics. Such questions as ''what proportion of ah cases of asthma are caused by occupational exposures?'' and ''is occupational asthma incidence rising?'' can now be answered. Surveillance programmes estimate the number of exposed individuals and the number of incident cases, allowing us to perceive the magnitude of the health problem. Although occupational asthma is a relatively common illness, very little is known about quantitative exposure-response relationships. Epidemiological techniques can quantify the importance of predisposing factors, such as atopy or cigarette smoking, in determining individual risk factors for occupational asthma. Still to be determined are the major, possibly genetic factors which make occupational asthma an ''idiosyncratic'' illness, affecting only a minority of those with equivalent exposures in the workplace. The goal of applying epidemiological techniques to the study of occupational asthma is ultimately to identify more effective means to prevent its occurrence.
