The objective of the study was to examine the effect of farming exposure, respiratory symptoms and smoking on the shape of the MEFV-curve in 1,691 male farming students and 407 male controls and to relate the slope ratio with FEV₁ and FEV₁ /FVC. Each subject underwent a medical interview and the slope ratios from the MEFV-curve at 75 (SR75), 50 (SR50) and 25 (SR25) %FVC together with FEV₁ and FVC were recorded. Histamine bronchial reactivity (Yan method) was measured and skin prick test with inhalant allergens was performed. In smokers, SR75 increased with increasing exposure to: general farming, swine and dairy cattle (p<=0.020). SR50 increased with increasing exposure to farming (p=0.015). In non-smokers, SR25 increased with increasing exposure to swine and dairy cattle (p=0.021) and increased SR25 was associated with sensitisation to house dust mite (p=0.017). Data revealed an interaction between smoking and exposure to farming. FEV₁ and FEV₁/FVC was not associated with farming exposure or production animals. FEV₁ and FEV₁/FVC (p<=0.003) were lower among subjects wi bronchial hyperresponsiveness and asthma (FEV₁ and asthma only in smokers). SR75 (p=0.037) and SR50 (p=0.024) were increased in subjects with asthma and SR75 was increased in subjects with bronchial hyperresponsiveness, but only in smokers (p=0.002). In conclusion, exposure to farming seems to influence the shape of the MEFV-curve and there are indications of interaction between exposure to organic dust and smoking. These changes are seen only in the slope ratios and not in FEV(1) and FEV(1)/FVC. However, FEV₁ and FEV₁/FVC are superior to slope ratios in differentiating healthy subjects from those with respiratory symptoms.