We measured response to 250 micrograms isoproterenol in 985 patients with COPD who were carefully studied and followed closely for nearly 3 yr. Response was quantitated in relative (% increase in FEV1) and absolute (change in FEV1 as a % predicted normal) terms. Patients were told to abstain from bronchodilator for 6 h before testing, and responses were larger in those who said they had done this. Responses were not related to blood theophylline concentrations. In patients who had abstained for 6 h, response averaged 15% of the baseline FEV1 or 5% of the predicted normal FEV1. Relative response was inversely proportional to baseline FEV1, whereas absolute response was directly related to baseline FEV1. Responses were positively related to symptomatic wheezing and exercise capacity, and negatively to smoking history, but these features accounted for little of the observed variation in response. Patients with large relative and absolute responses demonstrated increased variability of FEV1 with time and a decreased annual rate of decline of FEV1. There was little change in response with time, and the change observed could probably be explained by a gradual decrease in FEV1. However, time-related interindividual and intraindividual variations of response were large and impossible to separate from random variations of FEV1.