There are no population-based follow-up studies to estimate the fraction of asthma incidence that is attributable to work. In Finland, individuals with clinically well-established persistent asthma are registered for reimbursement of medication from the national health insurance scheme. We combined, at an individual level, these data with the population census data of 1985, 1990, and 1995 to estimate the attributable fraction of work in adult-onset persistent asthma. Our follow-up study covered the entire 25- to 59-yr-old employed population of Finland in 1986-1998. Relative risks (RR) for occupational categories were estimated in comparison to those employed in administrative work. There were 49,575 incident cases of asthma. The attributable fraction of occupation was 29% (95% CI 25-33%) for men and 17% (95% CI 15-19%) for women. The risk was increased especially in agricultural work, manufacturing work, and service work. In addition to already established risk occupations of occupational asthma, such as food and beverage work, the analysis identified a large number of occupations with significant excess of asthma incidence. The results indicate that the impact of occupational factors in the inception of adult-onset persistent asthma, and consequently the potential for prevention, is much larger and more widely spread than generally assumed.