Background A key feature of chronic obstructive pulmonary disease (COPD) is an accelerated rate of decline in forced expiratory volume in 1 second (FEV 1), but data on the variability and determinants of this change in patients who have established disease are scarce. Methods We analyzed the changes in FEV 1 after administration of a bronchodilator over a 3-year period in 2163 patients. A random-coefficient model was used to evaluate possible predictors of both FEV 1 levels and their changes over time. Results The mean (±SE) rate of change in FEV 1 was a decline of 33±2 ml per year, with sig - nificant variation among the patients studied. The between-patient standard deviation for the rate of decline was 59 ml per year. Over the 3-year study period, 38% of patients had an estimated decline in FEV 1 of more than 40 ml per year, 31% had a decline of 21 to 40 ml per year, 23% had a change in FEV 1 that ranged from a de - crease of 20 ml per year to an increase of 20 ml per year, and 8% had an increase of more than 20 ml per year. The mean rate of decline in FEV 1 was 21±4 ml per year greater in current smokers than in current nonsmokers, 13±4 ml per year greater in patients with emphysema than in those without emphysema, and 17±4 ml per year greater in patients with bronchodilator reversibility than in those without reversibility. Conclusions The rate of change in FEV 1 among patients with COPD is highly variable, with increased rates of decline among current smokers, patients with bronchodilator reversibility, and patients with emphysema. (Funded by GlaxoSmithKline; ECLIPSE ClinicalTrials.gov number, NCT00292552).