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 FEV(1) and FEV(1) /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< or =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. FEV(1) and FEV(1)/FVC was not associated with farming exposure or production animals. FEV1 and FEV(1)/FVC (p< or =0.003) were lower among subjects wi bronchial hyperresponsiveness and asthma (FEV(1) 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, FEV(1) and FEV(1)/FVC are superior to slope ratios in differentiating healthy subjects from those with respiratory symptoms.