The 7-item Asthma Control Questionnaire (ACQ) has been validated to measure the goals of asthma management as defined by international guidelines (minimisation of day- and night-time symptoms, activity limitation, beta(2)-agonist use and bronchoconstriction). Responses are given on a 7-point scale and the overall score is the mean of the responses (0=totally controlled, 6=severely uncontrolled). The aim of this analysis was to determine the cut-point on the ACQ that best differentiates between 'well-controlled' and 'not well-controlled' for (a) clinical practice (low risk of missing 'not well-controlled') and (b) clinical trials (low risk of including 'well-controlled'). All 1323 patients who provided data sets at week 12 in the Gaining Optimal Asthma Control (GOAL) clinical trial were included in the analysis. The gold standard for 'well-controlled' was a composite based on the GINA/NIH guidelines and derived from data collected in the clinical trial diaries and clinic records. The analysis showed that the crossover point between 'well-controlled' and 'not well-controlled' is close to 1.00 on the ACQ. However, to be confident that a patient has well-controlled asthma, the optimal cut-point is 0.75 (negative predictive value=0.85). To be confident that the patient has inadequately controlled asthma, the optimal cut-point is 1.50 (positive predictive value=0.88). In conclusion, knowledge of these cut-points will enhance practising clinicians ability to identify patients whose asthma requires additional treatment, enable investigators to enroll poorly controlled patients into studies and for both clinicians and investigators to evaluate whether treatment goals are being achieved.