The present authors aimed to determine if the criteria for the diagnosis of chronic obstructive pulmonary disease (COPD) and its classification by severity as recommended by the Global Initiative for Chronic Obstructive Lung Disease are supported by measurements of respiratory health-related quality of life. A community-based sample of adults aged 25-75 yrs had pre- and post-bronchodilator spirometry and completed the St George's Respiratory Questionnaire (SGRQ). Loess scatter plot smoothers of the SGRQ versus post-bronchodilator forced expiratory volume in one second (FEV(1))/forced vital capacity (FVC) ratio and post-bronchodilator FEV(1) % predicted together with receiver operating characteristic (ROC) curve analysis were used to determine the relationship between spirometric variables and clinically important differences in the SGRQ score. The scatter plot smoother and ROC curve analyses supported the value of 0.7 for post-bronchodilator FEV(1)/FVC ratio, which was approximately 4 units higher than the nadir of the SGRQ. To represent a distance of 8 units on the SGRQ, the cut-off points for post-bronchodilator FEV(1) that delimit COPD severity stages were 80, 60 and 40% pred for mild, moderate and severe COPD, respectively. To diagnose chronic obstructive pulmonary disease the use of post-bronchodilator forced expiratory volume in one second/forced vital capacity ratio of 0.7 is supported by health-related quality of life measurements. There may be advantages in using forced expiratory volume in one second cut-off points of 80, 60 and 40% predicted for the classification of mild, moderate and severe chronic obstructive pulmonary disease, respectively, similar to the approach recommended for asthma.