Our aim was to establish whether the case-control design may be applied in surveillance for occupational asthma. In a region with intensive manufacturing industries, we carried out one case-control study from 1974 to 1978 and another from 1989 to 1993; 325 and 387 pairs of cases of asthma and of age- and sex-matched control subjects, respectively, were selected. Current risks were found to be higher than past estimates for painters, farmers, millers/bakers, textile, wood/furniture, and chemical workers. High risks for asthma have recently appeared in leather, polyurethane plastics, hospital and food industry workers, welders, and shoemakers. It is concluded that the case-control approach may be used to describe variations in asthma risk by occupational categories and time. A method to measure the efficiency of the diagnostic process for occupational asthma is also proposed.