The Asthma Questionnaire is a 68-item quality of life scale designed to be sensitive to quality of life changes in clinical trials. The questionnaire covers 11 domains of life experience, the initial domain and item sets being derived from six qualitative focus groups of asthma patients. Psychometric analysis of responses of 101 asthma patients to the initial 101-item set showed the scale to be unidimensional despite being multi-domain, and the finding of unidimensionality was replicated during the further three stages of item refinement using 783 patients. The scale compensates for acquiescence bias as well as allowing a 'not applicable' response category. Validity of the scale was demonstrated by confirmation of expected group differences and the retest reliability was 0.948.