Objectives This study examined the determinants of asthma in a population of farmers, including as a crude indicator of genetic predisposition history of asthma in next-of-kin' (family history), and exposure factors such as animal production and smoking.

Methods In a cross-sectional study of 8482 farmers or farmers'spouses in Norway a questionnaire with information on asthma among the subjects and their next-of-kin, production type and farming activities, exposures outside farming, and smoking was applied.

Spirometry was performed.

Results The lifetime prevalence of self-reported asthma in the population was 6.3%. Significant risk factors for current asthma were asthma among next-of-kin, asthma as child or adolescent, animal production, and age.

In a comparison with subjects with no family history of asthma and no animal production the adjusted odds ratio for current asthma in never smokers was 1.9 [95% confidence interval (95% Cl) 0.4-8.9] for subjects with family history only, 2.2 (95% CI 1.1-4.2) for subjects with animal production only, and 6.3 (95% CI 3.1-13.1) for subjects with both factors.

A combination of animal production, smoking, and a positive family history gave an odds ratio of 8.1 (95% CI 4.0-16.2) for current asthma.

Conclusions The study can be interpreted as support for the hypothesis of an interaction between genetic factors and exposure factors in the causation of asthma. (...)