International guidelines on asthma management indicate that the primary goal of treatment should be optimum asthma control. The aim of this study was to develop and validate the Asthma Control Questionnaire (ACQ). The authors generated a list of all symptoms used to assess control and sent it to 100 asthma clinicians who were members of guidelines committees (18 countries). They scored each symptom for its importance in evaluating asthma control. From the 91 responses, the five highest scoring symptoms were selected for the ACQ. In addition, there is one question on beta2-agonist use and another on airway calibre (total questions=7). The ACQ was tested in a 9-week observational study of 50 adults with symptomatic asthma. The ACQ and other measures of asthma health status were assessed at baseline, 1, 5 and 9 weeks. In patients whose asthma was stable between clinic visits, reliability of the ACQ was high (intraclass correlation coefficient (ICC)=0.90). The questionnaire was very responsive to change in asthma control (p<0.0001). Cross-sectional and longitudinal validity were supported by correlations between the ACQ and other measures of asthma health status being close to a priori predictions. In conclusion, the Asthma Control Questionnaire has strong evaluative and discriminative properties and can be used with confidence to measure asthma control.