Objective: To assess whether the severity of asthma is associated with sensitisation to airborne moulds rather than to other seasonal or perennial allergens. Design: Multicentre epidemiological survey in 30 centres. Setting: European Community respiratory health survey. Participants: 1132 adults aged 20-44 years with current asthma and with skin prick test results. Main outcome measure: Severity of asthma according to score based on forced expiratory volume in one second, number of asthma attacks, hospital admissions for breathing problems, and use of corticosteroids in past 12 months. Results: The frequency of sensitisation to moulds ( Alternaria alternata or Cladosporium herbarum , or both) increased significantly with increasing asthma severity (odds ratio 2.34 (95% confidence interval 1.56 to 3.52) for either for severe v mild asthma). This association existed in all of the study areas (gathered into regions), although there were differences in the frequency of sensitisation. There was no association between asthma severity and sensitisation to pollens or cats. Sensitisation to Dermatophagoides pteronyssinus was also positively associated with severity. In multivariable logistic regressions including sensitisation to moulds, pollens, D pteronyssinus , and cats simultaneously, the odds ratios for sensitisation to moulds were 1.48 (0.97 to 2.26) for moderate v mild asthma and 2.16 (1.37 to 3.35) for severe v mild asthma (P Conclusions: Sensitisation to moulds is a powerful risk factor for severe asthma in adults. This should be taken into account in primary prevention, management, and patients9 education.