Individuals with CF greater than or equal to 10 years of age who are being started on intravenous (IV) antibiotics for a clinically diagnosed pulmonary exacerbation. Patients must demonstrate at least 3 of the 11 criteria for pulmonary exacerbation as defined by a Cystic Fibrosis Foundation (CFF) Consensus Conference. Treatment with a minimum of two IV antibiotics is required. We expect to enroll an approximately equal number of males and females. Most CF patients in our clinics are of white, non-Hispanic origin and we anticipate this ethnic mix to persist in this study. The majority of pediatric CF subjects will be admitted to the hospital for treatment purposes whereas many adults receive their IV antibiotics at home.