Background There are few longitudinal studies that have examined the association of lung function decline with indoor mould and dampness. Lung function decline in relation to dampness and mould in the home has studied in adults over a 9 year period. Methods Spirometry was performed twice in participants in the European Respiratory Health Survey (ECRHS I and II) who were initially examined aged 20-45 years, in 1990-1995 and 9 years later (n=6443). Information on their current home was collected twice by interview. Dampness (water damage or damp spots) and indoor mould, ever and in the last 12 months, were assessed. A dampness score and a mould score were calculated. In addition, 3118 homes at 22 centres were inspected directly at follow-up for the presence of dampness and mould. Results Dampness and mould were common. Overall, 50.1% reported any dampness and 41.3% any indoor mould in either ECRHS I or ECRHS II. Women with dampness at home had an additional decline in forced expiratory volume in 1 s (FEV(1)) of -2.25 ml/year (95% CI -4.25 to -0.25), with a significant trend in increased lung function decline in relation to the dampness score (p=0.03). The association in women was significant when excluding those with asthma at baseline. Observed damp spots in the bedroom was associated with a significant additional decline in FEV(1) of -7.43 ml/year (95% CI -13.11 to 1.74) in women. Conclusion Dampness and indoor mould growth is common in dwellings, and the presence of damp is a risk factor for lung function decline, especially in women