as in previous studies,7'8 we compared peak expiratory flow rates over seven day periods during respiratory episodes and control periods. By doing this we may have failed to recognise brief asthma exacerbations with modest decreases in peak expiratory flow rate, but we were less likely to include as exacerbations variability in airflow related to poorly controlled asthma.None of the previous studies in adults tested for coronaviruses, and Huhti et al did not culture for rhinoviruses.5 In our study, identifications of rhinoviruses by seminested reverse transcriptase polymerase chain reaction and coronaviruses by ELISA improved the diagnosis of both viruses and enabled us to show that they are important causes of respiratory disease leading to exacerbations of asthma in adults as well as in children.2A2425 Given the far better performance of polymerase chain reaction than conventional diagnostic techniques, the newer molecular methods should greatly facilitate research in this important area.This work was in part supported by the British Lung Foundation and the Cystic Fibrosis Trust. We thank Mrs L Jacklin for excellent technical assistance; Drs J Wales,