Based on data in the Finnish Twin Registry, which was obtained by postal questionnaire, the prevalence of chronic bronchitis among non-smoking farmers was 3.6% and among a corresponding group of non-farmers (reference subjects) 3.4%. The six-year incidences of chronic bronchitis for these 2 groups were 2.7% and 0.7%, respectively. The difference in incidence between these groups was significant (p less than 0.001). The fact that the incidence among farming subjects was three times the incidence among non-farming subjects indicates that chronic bronchitis is a work-related disease among farmers. This is analogous to previous findings that symptoms compatible with chronic bronchitis occur more often among grain elevator workers than among urban dwellers. The occurrence of chronic bronchitis among both farmers and grain elevator workers probably is associated with exposure to grain dusts.