Patients with cystic fibrosis who are being followed by the Medical College of Wisconsin or the University of Wisconsin-Madison are eligible for this study. Participants will have blood tests, pulmonary function tests, a sputum culture, and buccal swabbing (cotton swabbing of the inside of the cheek to collect cells for DNA study). In addition, their medical records will be reviewed for a history of lung infections and the results of various tests, including pulmonary function studies, chest X-rays and bacterial cultures. Blood samples collected previously at the Medical College of Wisconsin or the University of Wisconsin-Madison will also be analyzed for antibodies to bacteria.