To ascertain the prevalence and risk factors in isocyanate asthma, we made a mail survey in a sample of workers in the wooden furniture industry, which uses large quantities of isocyanate-paints. Firms making up the initial sample, chosen from a register in the Veneto region, were asked to cooperate in the study. The study population, selected from the respondent firms, consisted of 1,430 subjects (about 2% of the parent population). Each subject received a self-administered questionnaire on respiratory symptoms and occupational history; this was mailed to his/her workplace. Completed questionnaires were returned by 730 subjects, who made up 51% of the population under examination (about 1% of the parent population). Fifteen subjects had suffered their first attack of asthma while in their present job: of these 7 were among the 121 who used paints (5.8% prevalence), and 8 were among the 609 who had no exposure (1.4% prevalence). The risk of asthma in the former group was therefore almost 5 times the risk in the latter. This result, however, is probably an underestimation because of the cross-sectional design of our study; at the time of study many asthmatics had already left their factory. There was a dose-response relationship between asthma and the length of exposure to isocyanates, the prevalence increasing from 2.2% in subjects with less than 20 years to 27.3% in workers with over 20 years' exposure. Lower prevalence of isocyanate asthma was found in the larger firms, where airborne exposure to isocyanate was believed to be lower. The continuous mode of exposure increased the risk of developing symptoms of asthma rather than intermittent acute exposure to high concentrations of isocyanates. Among painters, the prevalence of asthma ranged from 7.1% in non-smokers to 2.9% in smokers. As smoking appears to increase IgE production, a nonallergic mechanism may underlie isocyanate-induced occupational asthma.