We tested the hypothesis that individuals heterozygous for the common cystic fibrosis delta F508 mutation are at risk of obstructive pulmonary disease. We studied a cross-sectional sample from the general population of Copenhagen, aged 20 years and older. We performed spirometry to measure FEV1 and FVC, and genotyped blood samples from 9141 individuals. We identified 250 carriers of the delta F508 mutation (2.7%; 95% CI: 2.5%-3.1%). Nine precent of carriers reported having asthma compared with 6% of non-carriers (chi 2: p = 0.04). Furthermore, among individuals with airway obstruction, the percentage of predicted FEV1 and FVC were significantly lower in participants heterozygous for delta F508 than in non-carriers (49% vs. 58%, p = 0.004 and 70% vs. 82%, p <0.001, respectively). Cystic fibrosis delta F508 heterozygosity may be over-represented among people with asthma and seems to be associated with decreased pulmonary function in people with airway obstruction who also have asthma.