Second only to sickle-cell anemia, cystic fibrosis is the most common genetic disease causing early death (1). Although pulmonary disease causes most of the morbidity and mortality associated with cystic fibrosis, the lungs are thought to be anatomically normal at birth (2). In patients with cystic fibrosis, progression of lung disease is insidious, and patients may be relatively asymptomatic before irreversible changes and chronic bacterial colonization occur. Cough is the predominant symptom in the early stages of cystic fibrosis, occurring in as many as 50% of patients by 10 months of age (3); however, many patients do not have pulmonary symptoms. The first detectable evidence of lung disease in patients with cystic fibrosis is infection and/or inflammation in bronchoalveolar lavage fluid, denoted by elevated counts of interleukin-8 and neutrophils and the presense of microorganisms (4–7). However, detecting bacteria is complicated by regional heterogeneity of inflammation and infection (8–9). Haemophilus influenzae, Staphylococcus aureus, and P. aeruginosa are the most prevalent early pathogens, and most patients have colonization with at least 1 of these bacteria by 1 year of age (10–11). Early infections with P. aeruginosa can be transient, and approximately half clear spontaneously (12–14). However, by their teenage years, most patients have colonization with P. aeruginosa(11).