Individuals with cystic fibrosis (CF) develop chronic lung infections and suffer intermittent acute exacerbations of their lung disease. Most exacerbations are not treated until they cause increased symptoms, and patients seek medical attention. This proposal details a study of home lung function and symptom monitoring. Subjects will be randomly assigned to one of two groups: 1) home monitoring, in which spirometry and symptoms are recorded; or 2) standard care. The home monitoring data will be transmitted electronically to the study center. If spirometry or symptoms have deteriorated substantially, treatment for a CF pulmonary exacerbation will be initiated. It is anticipated that use of home monitoring will lead to earlier, more reliable recognition and treatment of exacerbations, which will translate into better lung health.