BACKGROUND: Several epidemiological studies have reported a higher prevalence of respiratory symptoms in subjects living in damp housing, but links with specific respiratory diseases such as asthma have not been satisfactorily established. METHODS: One hundred and two subjects with physician diagnosed asthma and 196 age and sex matched controls were interviewed; 222 (75%) then agreed to have their dwelling surveyed for dampness. The prevalence of both self-reported and observed dampness in the homes of the asthmatic subjects and controls were compared. Both asthma and the severity of the dampness were quantified so that the possibility of a dose-response relationship could be investigated. RESULTS: Asthmatic subjects reported dampness in their current (odds ratio (OR) 1.92, 95% confidence interval (CI) 1.18 to 3.12) and previous (OR 2.11, 95% CI 1.29 to 3.47) dwellings more frequently than control subjects. The surveyor confirmed dampness in 58 of 90 (64%) dwellings of asthmatic subjects compared with 54 of 132 (41%) dwellings of control subjects (OR 2.62, 95% CI 1.50 to 4.55). This association persisted after controlling for socioeconomic and other confounding variables (adjusted OR 3.03, 95% CI 1.65 to 5.57). The severity of asthma was found to correlate statistically with measures of total dampness (r = 0.30, p = 0.006) and mould growth (r = 0.23, p = 0.035) in the dwelling. Patients living in homes with confirmed areas of dampness had greater evidence of airflow obstruction than those living in dry homes (mean difference in forced expiratory volume in one second (FEV1) 10.6%, 95% CI 1.0 to 20.3). CONCLUSIONS: Asthma is associated with living in damp housing and there appears to be a dose-response relationship. Action to improve damp housing conditions may therefore favourably influence asthma morbidity.