Individuals with CF develop chronic lung infections and suffer intermittent exacerbations, which require intensive treatment with antibiotics. The most common and useful objective measure of CF lung disease is spirometry. Chronic treatment of CF lung disease requires airway clearance, mucolytics and antibiotics. These treatments have been quite successful and there is evidence that early, aggressive treatment of lung disease results in better outcomes. Unfortunately, most exacerbations are not treated until they cause pronounced deterioration in symptoms, which prompts patients to seek medical attention. Self-monitoring of clinical status has improved outcomes in many other disorders such as asthma, diabetes mellitus, and lung transplantation. This is an important, randomized trial of home lung function and symptom monitoring in CF. Subjects will be assigned to one of two groups: 1) Home monitoring, in which spirometry and symptoms are recorded daily; or 2) Standard Care. The home monitoring data will be transmitted electronically twice weekly to the study center, where the results will be reviewed. If spirometry or symptoms have deteriorated substantially below baseline, treatment for a CF pulmonary exacerbation will be initiated. It is anticipated that use of home monitoring will translate into better clinical outcomes. We will test the hypothesis that if pulmonary exacerbations are identified and treated earlier than the current standard of care, the progression of lung disease will be slowed.