To establish the correlation among asthma efficacy parameters over a long period, data from over 1,500 patients in two one-year asthma clinical trials with montelukast, a Cys-LT1 antagonist, were analysed. Airway obstruction measurements, forced expiratory volume in one second (FEV1) and peak expiratory flow (PEF), were measured at clinic visits. Patients recorded daytime symptom score, "as-needed" beta-agonist use, and PEF on a daily basis. Relationships among these parameters at baseline and during the one-year treatment period were established by correlation analyses. Multiple correlations between the airway obstruction (FEV1 and PEF) and patient-reported measurements were evaluated by canonical correlation analysis. Pairwise correlations of the efficacy parameters over a one-year time period were stable. Canonical correlation between the airway obstruction and patient-reported asthma efficacy endpoints was low, indicating that each category of endpoints measures a distinctively different aspect of the disease. It appears that at least one endpoint from each category should be used in asthma clinical studies.