The aim of this study was to evaluate the measurement properties of an Asthma Quality of Life Questionnaire. The study design was an 8-wk unblinded single cohort with assessments at 0, 4, and 8 wk. Thirtynine adults with symptomatic asthma and a wide range of airway responsiveness and medication requirements were enrolled from previous studies and through notices in the local media. Those with inadequately controlled asthma were offered an inhaled steroid (budesonide). Asthma Quality of Life Questionnaire, spirometry, a clinical asthma control questionnaire, medication requirements, airway responsiveness to methacholine, the Sickness Impact Profile, and the Rand questionnaire were recorded at each visit. Patients kept daily diaries of peak flow rates and medications. The Asthma Quality of Life Questionnaire was able to detect changes in patients who responded to treatment or who had natural fluctuations in their asthma (p < 0.001) and to differentiate these patients from those who remained stable (p < 0.001). The Questionnaire was reproducible in patients who were stable (intraclass correlation coefficient = 0.92). There were significant longitudinal and cross-sectional correlations between asthma quality of life and other measures of both clinical asthma and generic quality of life. We conclude that the Asthma Quality of Life Questionnaire has good measurement properties and that it is valid as both an evaluative and a discriminative instrument. It measures the component of asthma most important to patients, and it should be considered for inclusion in all asthma studies.