Background. Cystic fibrosis is a recessive disorder mainly characterised by lung disease. We tested the hypothesis that individuals heterozygous for the common cystic fibrosis ΔF508 mutation are at risk of obstructive pulmonary disease. Methods. We studied a cross-sectional sample from the general population of Copenhagen, Denmark, aged 20 years and older. We did spirometry to measure forced expiratory volume in 1 s (FEV1) and forced vital capacity (FVC), and did genotyping on blood samples of 9141 individuals. We asked all participants whether they had asthma, and asked for information on smoking and other factors that could have contributed to obstructive pulmonary disease. Findings. We identified 250 carriers of the ΔF508 mutation (2.7% [95% CI 2.5-3.1]). 9% of carriers reported having asthma compared with 6% of non-carriers (p = 0.04). The odds ratio for asthma in participants heterozygous for ΔF508 mutation was 2.0 (1.2-3.5, p = 0.02). Furthermore, among with airway obstruction, the percentage FEV1 and FVC were significantly lower in participants heterozygous for ΔF508 than in non-carriers (49 vs 58%, p = 0.004; and 70 vs 82%, p < 0.001, respectively), mainly due to an effect in those with self-reported asthma. Interpretation. Cystic fibrosis ΔF508 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.