Pulmonary Function Tests (PFTs) are used to measure and track lung health of patients with CF. Performing a PFT takes practice and requires patients to perform certain breathing maneuvers consistently. Infants and young children cannot be tested using spirometers. The investigators start PFT training with our patients after they reach 4 years of age. It may take several months to a few years for useful, consistent and interpretable PFTs to be obtained. According to standards developed by the American Thoracic Society, children must be able to breathe in very deeply, exhale fast and hard, and continue exhaling for several seconds. Children also must perform these maneuvers in a repeatable manner. (Ferris, 1978 ). Recent software programs have been developed to encourage and assist children in completing the breathing maneuvers. Videogame-like graphics respond to children's efforts. Children can often choose which "game" to play, such as, "Blow out the candles."