In an ordinary Norwegian population which we studied, one in 13 persons suffered from an obstructive lung disease--bronchial asthma or chronic obstructive lung disease. The prevalence of the disease was the same in both sexes, but increased with age. There was a strong association between smoking and the disease. Occupational airborne pollution was an important risk factor. Taking the longest job held as a basis for the study, the adjusted odds ratio for obstructive lung disease in those exposed to a high degree of airborne pollution was 2.5 relative to those not so exposed. There was no difference between urban and rural areas in the frequency of the disease. Allergy and bronchial hypersensitivity were both associated with obstructive lung disease. Greater efforts should be put than at present into preventing obstructive lung disease.