Diisocyanates have been the most commonly identified cause for occupational asthma (OA) in industrialized areas. Asthma among diisocyanate workers may be true occupational asthma, caused by a high level of irritant exposure at work or by sensitization to diisocyanates. Alternatively, asthma may be coincidental to or may be aggravated by work exposures. A clear diagnosis usually requires a combination of investigations (serial peak expiratory flow recordings, methacholine challenges, and/or diisocyanate challenges), but it is important to provide the best management by identifying whether workplace changes are needed for the worker. Preventive measures to reduce the risk of occupational asthma from diisocyanates have not been prospectively evaluated. The introduction of a medical surveillance program (in Ontario, Canada) in 1983 was followed by retrospective assessments to determine benefits. Between 1980 and 1993, the proportion of all accepted compensation claims for OA that were attributed to diisocyanates, classified by year of symptom onset in the province with the program, rose to 64 percent by 1988, then fell significantly down to 29 percent in 1992 and 35 percent in 1993. Besides the medical surveillance program for diisocyanates, possible factors contributing to this reduction may include reduced diisocyanates exposures and better awareness of diisocyanate-induced asthma, both by workers and physicians. Compared with OA caused by other agents, those with OA due to diisocyanates had a significantly earlier onset of asthma after the start of the exposure (mean 5 yr vs. 7 yr), were younger and less likely to be atopic and to have smoked. The mean duration of asthma before the main medical assessment for compensation was significantly shorter among those with diisocyanate-induced asthma (mean 2 yr vs. 3 yr), and the severity was milder as assessed by medication use and pulmonary function. Those with diisocyanate-induced asthma were significantly less likely to be hospitalized for asthma. Among the subset whose outcome was determined at a mean of 2.1 years after the main medical assessment, the outcome severity was less for those with diisocyanate-induced OA. Among those with diisocyanate-induced OA, an earlier diagnosis and a trend to better outcome was found in workers from companies that were identified to be in compliance with surveillance measures.