Because toluene diisocyanate (TDI) is a strong sensitizer for asthma among workers in polyurethane factories, it is mostly extracted from the factory premises. The influence of such emissions on the prevalence of asthma among the people living in the vicinity of a polyurethane factory in Kouvola, Southern Finland, was studied through a questionnaire survey sent to 6807 persons living around the factory and in a control area; of these 4182 (61%) responded. In the study area near the factory there were 68 cases of asthma out of 3153 respondents (2.2%). In the control area there were 25 cases out of 1029 respondents (2.4%). The difference is insignificant (chi 2 = 0.27). Among the middle-aged the prevalence was significantly higher in the control area (chi 2 = 6.8). There was some indication of a lower asthma prevalence in the zone nearest to the factory, possibly due to its psychologically repellent effect on asthmatics, causing them to move away. Serum samples from 62 asthma patients out of 68 contacted (91%) were received and analyzed for TDI, HDI, and MDI. A positive result for the isocyanates was observed in only one patient who had been exposed in his occupation outside the factory. It was concluded that the polyurethane factory did not have a noticeable influence upon the prevalence of asthma in its surroundings.