This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The conduct of clinical trials in infants with CF has been hindered by the lack of sensitive and reproducible outcome measures. In older subjects, spirometry is the gold standard for quantifying obstructive lung disease. Because infants cannot perform such a maneuver, measurements in the first several years of life have been restricted to expiratory flows obtained in the tidal volume range, where they are highly variable and poorly reproducible. Recently, techniques have been developed for measuring forced expiratory flows from raised lung volumes in infants and young children, for the first time producing flow-volume curves in infants comparable to those obtained by spirometry in older subjects. These new techniques have the potential to dramatically improve the understanding of the early natural history and pathophysiology of CF lung disease, and serve as reproducible outcome measures for clinical trials. Each subject will undergo testing at enrollment (Visit 1), 6 months and 12 months after enrollment (Visits 2 and 3), and at a study visit to be scheduled one month after visit 1 (the One-Month Reliability Visit). To ensure that subjects are not experiencing an intercurrent illness, testing at 6 and 12 months after enrollment may be carried out up to one month before or after each of these scheduled visits. The One Month Reliability Visit should be scheduled 28 to 35 days after Visit 1 (Month 0). If testing in that time frame is not feasible, the One-Month Reliability Visit may be re-scheduled 28 to 35 days before or after Visit 2 or 3 The goal of the proposed study is to test the hypothesis that measurements of fractional lung volumes and forced expiratory flows generated by the new raised volume pulmonary function testing technique will serve as useful outcome measures for clinical trials in infants with CF.