A prospective patient cohort will be studied extensively physiologically, functionally, and immunologically upon enrollment while in the stable state. As part of the study, participants will be trained in the use of peak flow meters, so that they can record daily first morning peak expiratory flow rates (PEFR). To confirm the range of fluctuations in their basal state, participants will be then be followed at three-month intervals for face-to-face interviews and more limited physiological and functional testing described below. Participants will also be reminded at each scheduled visit to contact the study coordinator when they feel that an AE-COPD may be present. If they do contact the study coordinator, they will be evaluated at one of the study sites within 48 hours.