An outbreak of occupational asthma, of unknown cause and extent, was detected in a steel coating plant. In 1979 a cross-sectional study which defined occupational asthma in terms of respiratory symptoms detected 21 people with suggestive symptoms among the 221 studied. They all worked in the coating shop, but the plastic coatings used at the plant contained many potential sensitising agents that might have caused the asthma. All 21 developed their symptoms after 1971, and it was found that in this year a supplier had modified a coating allowing, at the temperatures used in the process, toluene di-isocyanate to be liberated. Two of the symptomatic subjects were tested by inhalation of the isocyanate and showed asthmatic reactions and other subjects were found to have asthma related to periods spent at work by records of peak expiratory flow rate. Over half the 21 had a symptom free latent period after first exposure of three years or less, a pattern not seen in other subjects with respiratory symptoms. After the isocyanate had been removed from the process 17 of these subjects became asymptomatic or improved, a greater proportion than in other subjects with respiratory symptoms.