In order to study the role of occupational exposure in the etiology of asthma, 78 asthmatics and 56 nonasthmatics from the Finnish twin cohort were investigated by means of a postal questionnaire. Among those studied were 31 identical twin pairs discordant with regard to asthma (i.e., only one member of the pair had asthma). The questionnaire inquired into the diagnosis and status of the asthma, smoking habits, atopic background, smoking history of the parents, and history as regards pets, and requested a detailed description of occupational exposure to airway allergens and irritants. Classification into asthmatics and nonasthmatics was based on the information gathered with the questionnaire, supplemented by other information whenever possible. Estimation of exposure was based on the subject's own report, on the work descriptions, and on the general knowledge about the exposure levels associated with work tasks in question. Exposure to organic solvents was found only in the asthmatic members of the discordant pairs, and none of the nonasthmatic persons had been exposed to solvents. There were no statistically significant differences as regards exposure to other unspecific irritants. Combined exposure to organic allergens and airway irritants was more common in the asthmatics than in the nonasthmatics (P = 0.009). Exposure to irritants was also more common among the asthmatics than the nonasthmatics with similar exposure to organic allergens (P = 0.004).